Trying to cap the power level of a weapon using ammo solves the long term problem, but not the short term. As an extreme example, imagine a situation where I'm spawning with a Rocket Launcher that kills all the enemies on the map, but it can only be fired once. Sure, that's only going to happen once a match, but the one time it does happen is going to be frustrating for everyone involved. Ultimately though if you just try to directly counteract the power of a weapon with the amount of ammo availible for it, people will eventually find the right balance of power and longevity via ammo and that will become the dominant weapon. It's better to just let weapons be strong and have matchmaking heuristics to take into account the power level of player's gear when making matches.