Well okay look I think not having a draw back does not make a character bad outright. But I think the Blue Marvel's problems come from a lot of factors.
1. The continuity issues he brings up, like Marvel has a superhero history that goes beyond just the golden age then modern age and the truth? Early writers introduced black superheroes who operated in the 1950's. Because well they want to have more diverse characters in the past. So his excuse to quit is kind of lame.
2. Marvel has so many superman character besides the Sentry. Some you never heard of. It also hurts that Icon from DC/Milestone already did the whole black Superman thing first.
3. His miniseries was not the best written comic, and this matters a lot. When you introduce someone while they might get better after showing up getting a title right off the bat with your name and is mediocre means most people are not going to care.
I like what Ewing did with the Blue Marvel, but when your first story for Marvel is how you quit for DECADES AND NOW show up, he ends up looking like an asshole no matter what.