I hear ya. Sentinels are my favourite AT. I've been reading about how they're underpowered being a cross between a tank and blaster/scrapper/brute. But I personally like how well rounded they are--and honestly how underpowered they are. I'm not the kind of guy that goes for the strongest build. Quite the opposite. I really enjoy the handicap. I suppose it goes with the story or immersive part of being a hero.
I tried playing around with other ATs, and found it a little too hard to suspend my disbelief. Yes, they have powers, some godlike powers, but I feel they're powers define them more than the person they are (Ironman man vs suit). Some of the toons I tried were fun (or funny), but they felt like they were trying too hard to be a hero for the sake of the cape. Sentinels feel real to me and much more believable. (I suppose this is incredibly subjective to my personal game play style, though). It reminds of the Saitama's conversation with King. Being a hero isn't about being the strongest or getting satisfaction out of fighting, it's about one's unwavering ideals of justice, fighting to save others, and most of all having the courage to do what must be done (even when you're out of your league).
ATM, I'm having a blast switching between two toons: a DP/WP (who's basically a guy with a gun trying to play vigilante in a world of super-powered folks) and a Archer/Bio (a mutation with hard skin and a bow and arrow -- reminds of Hawkeye in Age of Ultron). They're both too weak to take on mobs at any given time, and they both don't have enough damage to single shot an enemy, let alone a boss. Both of them are pretty much outclassed everywhere they turn, but that's the part I like. You're a hero, you find ways around your weakness or inadequacies to save the day.