I have to agree, the MGS series have certain gadgets and gameplay elements that are kind of OP or broken, depending on how you look at them. I love the series but I've never really diversified my approaches just because I could if I wanted to. It was always about finding the safest, easiest and if possible, quietest way of clearing a room. And this would most often entail using the tranq gun and disposing of enemies in the quietest way. I guess that's just the type of player I am, I tend to sweep and clean locations, disable guards even if I don't have to, or at least the ones that are in the way.
It's very cool that MGS lets you approach things differently and it's certainly a valid game design philosophy, but why would I jump on one leg while doing it if the game lets me exploit it? That there's a different philosophy, that the game should force, or at least encourage you to try a different approach, because the first, exploitable one, doesn't work very well anymore. It's basically temporarily taking away certain gameplay elements in favor of others to balance it out.
An example would be to have a large number of missions where enemies are say, fully armored or full of nanomachines that are resistant to your tranq gun, so you can't headshot them with it or put them to sleep that way. The OP tranq gun is now out of the picture so you need to think on your toes and try to distract the guards, put them down with CQC (which could be more complicated in this case as in more dangerous), create a diversion or evade them completely by learning their movement patterns.
I guess what I'm saying is that the tranq gun and fultoning can be extremely OP, especially if you're a careful, resource hogging player and thus can make most of the freedom redundant for a lot of players.
And this is true for most stealth games I've played, like Splinter Cell and Hitman. There's always a silenced gun, tranquilizer or not, strangling or putting to sleep and dragging or carrying someone out of the way. Mind you, it's extremely enjoyable for me to do these things but at the end of the day, when I stop and think about it, I've basically done that one same motion throughout the whole game, not even using most of the weapons, distraction gadgets and such. Why leave a problem unresolved (evading, distracting) when you can permanently take it out of the equation.
Payday 2 mixes things up a bit where you can knock out/kill a maximum of 3-4 guards, and only if you answer their radios and you can also subdue some by intimidating them. In other words, the hardlock they number of fultoned, tranqued or killed guards so you can't exploit it forever. It's not that well implemented in PD2 mind you, but I like where they were going with it.
I'm more for the idea that a game should be designed to guide and encourage the player to fully experience all facets of gameplay.