Back on topic...
Ember: Honestly, I think I see what they were trying to do with her- pure damaging skills that fire right off the bat, maybe meant to interrupt and screw over other heroes with fast skills- but it's pretty much ruined by her awful ultimate and her lack of firepower. She'll need a buff and/or a REALLY GOOD awakening skill to come back swinging.
Shirley: I desperately want her to be good because she's a disgustingly cute Pudge and her skills look fun on paper, but she kinda falls into the trap of being a super specialized tank that's chewed up by damage types she DOESN'T specialize in. Also, her lifestealing skill can be interrupted, in exchange for how it locks in one frontliner.
Vincent: I see Awakened Vincent almost exclusively on the higher levels of CoF, and I swear people are putting him on their teams just to screw with anyone trying to heal in CoF.
Boomer: He never struck me as an awful hero, just... one that can't stack up to everyone else, through no fault of his own. I'm curious what could actually be done to make him good, or at least competitive.
Esabel: On the game's release, I remember that they had to BUFF her, by making it so her ult temporarily increases her defenses. It didn't work.