Nah, in my opinion, Sent had a lot more than an assist and level 3 X factor going for him. He can zone, potentially punish people who guess wrong with relatively low risk/high reward armored attacks, decent range, his ability to control the air, and, on top of that, drone super is fuckin' bananas even outside of combos (especially for baiting assists) as even if you hit him, they still come out, setting you right back to step 1. The fact that he out-damages most of the cast with relatively safe stuff while also being able to take more hits than all of the cast certainly helped to make him a good character. This way, at least, once you DO guess right against Sentinel , you can get real damage on him without having to spend all your characters against him and then having to worry about the other two guys and he still remains a decent character.
Also, I always thought it kinda dumb that Sentinel is/was better than Hulk in damn near each and every way.