I played Splinter Cell: Blacklist for the first time recently - running through some coop missions with a friend - and he, having played it before, advised me to be a bit careful with some of the upgrades you can purchase for your character, since they can make the game far too easy. It's been a while since I played a Splinter Cell (the third in the series might have been the most recent one), and while things have changed a fair bit, the basic gameplay principles were easy to pickup, and we had a lot of fun. But even as a relative newbie, looking through some of the upgrades it was easy to see what he meant.
Before going any further, I'll reiterate that I've only played coop missions so far, so the following comments likely don't apply to the single player game in quite the same way, though from what I've read and heard it sounds like they will do at least to a point. We were also playing on Perfectionist difficulty, which disables an 'Execute' mechanic allowing you to quickly and automatically kill enemies you've tagged (tagging itself is still useful in Perfectionist mode to track where they are and let your teammate know about them).
The biggest immediate problem is the weapon upgrades you can get. With the basic loadout, you have a silenced pistol and a silenced SMG, and about two clips for both. They aren't great weapons, especially against enemies with armour or helmets. When we got spotted, and more than three or so enemies rushed one of us, or if we were out in the open at the time, we would need to run or we would die. It was more effective to hide and distract them while your teammate could hopefully sneak around and stealth attack them from behind than to get into a firefight. This seems entirely appropriate for a stealth game. Then I bought an assault rifle.
The biggest immediate problem is the weapon upgrades you can get. With the basic loadout, you have a silenced pistol and a silenced SMG, and about two clips for both. They aren't great weapons, especially against enemies with armour or helmets. When we got spotted, and more than three or so enemies rushed one of us, or if we were out in the open at the time, we would need to run or we would die. It was more effective to hide and distract them while your teammate could hopefully sneak around and stealth attack them from behind than to get into a firefight. This seems entirely appropriate for a stealth game. Then I bought an assault rifle.
I limited my use of it, since it wasn't silenced and we were aiming for stealth, but based on how it performed when things got loud, I could probably have gone Rambo through most of the levels on my own (well, it would have required a little more care than that, but you know what I mean). This was compounded by a suit upgrade which gave me more ammunition. This wasn't even the best assault rifle on offer or a fully upgraded suit - if I invested in the top weapons and a fully combat focused suit, I'm certain I'd be pretty much unstoppable. Given the missions we played, where being spotted brings in maybe 10-15 more enemies on top of the maybe 10 or so initially in the level, there wouldn't be all that much threat. It would be overkill to the extreme.
There are lots of other, more stealthy, upgrades, but I can see them going to the same point. You start with a smoke grenade, which can cover your movement or provide a bit of protection if you get spotted and need to relocate. The first upgrade will disable enemies in the smoke, making them easy pickings, and then the second upgrade makes it knock them out. That's a pretty steep escalation of power. Suddenly, you don't even need to get close to enemies, you can just toss gas at them and walk past their snoozing bodies.
On the other hand, it's also possible to make a game where your upgrades don't really feel like they make much difference. It's easy for RPGs to fall into this trap, where your +1 strength and extra damage sounds cool, but comes around at the same time as tougher enemies with more health, ultimately maintaining the status quo. Maybe you get a new ability to stick enemies in place or set them on fire, and that's novel, but generally you're still going to have to hit them with your weapons to finish them off. I'll say this for the Splinter Cell upgrades, I definitely felt more badass after getting a few. And we were playing the stealthy missions, rather than the more combat oriented ones where you need to defend against waves of enemies, so perhaps I'll be eating my words about being unstoppable once we try those.
There are lots of other, more stealthy, upgrades, but I can see them going to the same point. You start with a smoke grenade, which can cover your movement or provide a bit of protection if you get spotted and need to relocate. The first upgrade will disable enemies in the smoke, making them easy pickings, and then the second upgrade makes it knock them out. That's a pretty steep escalation of power. Suddenly, you don't even need to get close to enemies, you can just toss gas at them and walk past their snoozing bodies.
On the other hand, it's also possible to make a game where your upgrades don't really feel like they make much difference. It's easy for RPGs to fall into this trap, where your +1 strength and extra damage sounds cool, but comes around at the same time as tougher enemies with more health, ultimately maintaining the status quo. Maybe you get a new ability to stick enemies in place or set them on fire, and that's novel, but generally you're still going to have to hit them with your weapons to finish them off. I'll say this for the Splinter Cell upgrades, I definitely felt more badass after getting a few. And we were playing the stealthy missions, rather than the more combat oriented ones where you need to defend against waves of enemies, so perhaps I'll be eating my words about being unstoppable once we try those.
One option here is to simply restrict yourself - if you know packing an assault rifle and insta-disable gas makes the mission easy, then just don't take them. The game already offers bonus points if you can complete missions without disturbing the guards at all, and we did a few missions where you needed to avoid being spotted that obviously required a certain amount of stealth (though you could still disable guards to stop them spotting you). The counterpoint to that might be that surely the designer has some kind of vision, and it doesn't hurt to enforce it somewhat. Perhaps they could have said 'no alternate weapons' (meaning the SMGs, assault rifles, etc) and no deadly gadgets (so no frag or incendiary grenades) for stealthy missions. You can still take a heavily upgraded combat pistol if you want, but it's gonna be that much harder if combat becomes necessary. These restrictions could be made an option for the host to disable if they like - sometimes it is fun to just take on a mission in a totally inappropriate or overpowered way - but it would reduce the chances of people sabotaging their own fun by mistake, especially at the point when they're first experiencing the game. Give pros and those who know what they're looking for the options to mix up the gameplay however they like, but shepherd newer players towards a good default.
This has also sometimes been an issue in certain RPGs, where there are 'good' character traits and stats, worth spending your limited points on, and 'bad' ones, which are very much not. Maybe botany or alchemy sounds like a cool thing to be able to do, but in fact it serves very limited purpose, and is no use at all in combat - which, let's be honest, is often where you'll be spending a lot of your time. Or like the notorious example of Deus Ex: Human Revolution, where you could define yourself as a stealthy hacker, only to find yourself inevitably locked in a small room with a huge mini-gun toting cyborg as part of a plot mandated boss battle (I still can't believe those boss battles made it into the final game in the form they did - luckily I took some combat skills, but boy would it have been frustrating if I hadn't...)
It's hardly fair to allow someone, who has never played the game before and has no familiarity with the skills on offer or the challenges they'll be facing, to screw themselves over before they've even encountered their first giant rat. That's why it's good to see games offer default starting loadouts for those disinclined to read up on the options in advance, while retaining the option for experienced (or foolhardy) players to set up a totally custom character. In short, I don't think players should be easily able to either totally overpower or totally hobble themselves through their upgrade or development choices - unless they go out of their way or setup a custom game to do so.
This has also sometimes been an issue in certain RPGs, where there are 'good' character traits and stats, worth spending your limited points on, and 'bad' ones, which are very much not. Maybe botany or alchemy sounds like a cool thing to be able to do, but in fact it serves very limited purpose, and is no use at all in combat - which, let's be honest, is often where you'll be spending a lot of your time. Or like the notorious example of Deus Ex: Human Revolution, where you could define yourself as a stealthy hacker, only to find yourself inevitably locked in a small room with a huge mini-gun toting cyborg as part of a plot mandated boss battle (I still can't believe those boss battles made it into the final game in the form they did - luckily I took some combat skills, but boy would it have been frustrating if I hadn't...)
It's hardly fair to allow someone, who has never played the game before and has no familiarity with the skills on offer or the challenges they'll be facing, to screw themselves over before they've even encountered their first giant rat. That's why it's good to see games offer default starting loadouts for those disinclined to read up on the options in advance, while retaining the option for experienced (or foolhardy) players to set up a totally custom character. In short, I don't think players should be easily able to either totally overpower or totally hobble themselves through their upgrade or development choices - unless they go out of their way or setup a custom game to do so.