I don't think Sentinel is broken/overpowered but he is at least upper tier. On paper no matter how you dice it, he is a powerful character.
PROs:
*Highest stamina in the game
*2nd highest average damage output (1st being Hulk)
*Has Flight mode for increased mobility adding extra flavor to his combos and air game.
*Normals have huge hitbox. Stomps and frying pan still cover a lot of screen.
*Super armor on some normals and launchers
*HSF is generally safe on block and can punish from full screen. Continues OTG combo from Low Rocket Punch.
*Decent block damage/chip game as a lot of his moves do chip damage
*One of the best assists in the game. Perfect for rushdown teams
*One of the highest boosts in damage from X Factor
*Really fast dash on the ground
*Command throw that can be comboed off of
*Air hyper Drive (forgot its name) is completely invulnerable and beats everything (just about)
*In corners Sentinel's hypers combo into each other super easily for insane damage and combos that usually kill off a character.
*As far as his ranks compared to other "heavy" characters go, he is far more mobile and can zone better than most of them (compared with She Hulk, Hulk, Thor, Tron, Haggar). He is not as assist dependent nor is his game dependent on getting up close for grabs or putting opponents into corner. He is solid all around.
CONs:
*Poor anti-air game. Sentinel cannot deal effectively with anything coming from the skies as he has no genuine anti-air move.
*While he has an amazing assist, it keeps him out for a very long time and if he gets hit the drones are cancelled. This is the time when it's the easiest to kill a full health Sentinel.
*Extremely large size means landing combos are easy on him and every infinite/loop/combo is going to work on him. Same thing for instant overheads.
*Low mix up game. It is difficult for Sentinel to "open up" a tight defense without assists. A good defense that is also good at punishing mistakes easily beats Sentinel.
*Defensively, Sentinel is exposed to crossovers, mix ups, set ups and a lot of shenanigans in the game. During Flight mode he can't block so he is most vulnerable there.
*Sentinel isn't the fastest, most mobile, best grappler, best zoner, best rushdown or best "comeback" character in the game.
He is however the most mobile tank in the game and a character with dangerous comeback potential as an anchored assist character.