I have killed rupture spitters with melee and survived. A boiler on the other hand, can kill a whole group of marines even when they are not within melee distance. Also, a boiler can control when it explodes.
Rupture spitters are dangerous but they aren't overpowered, IMO. Marines have access to smartguns, sentry turrets, etc. The gameplay is asymmetrical so it isn't balanced the same as your average FPS.