I don't think that the companions needed more backstories, imo they needed to be more involved in the game. I think all companions quests need 'begining - middle - end" in their stories and in most cases this should have been more integrated into the main story. Cullen's mini quest with lyrium is the perfect example of it done right.
I'm fine with Bull being "human" it doesn't take the Inquisition for him to have his doubts, that happened long before that and he out to be comfortably "human" because he is a spy anyways. Him being hedonistic is an expression of his Qunari side, it becomes more obvious if you romance him. What did got lost in translation was that Bull was there to spy and make sure that the Qunari goals were met, it all got too focused on his personal problem. Another important flaw in Bull's quest is that the battle itself and thread to charges doesn't feel very important. The problem is the "choreography", it is especially hard to put your disbelieve aside when you see a handful of Venatori walking casually up the hill, forced drama right there.
Cassandras quest is too short, you never really deal with consequences, half of it is done through WT and you basically just run in to kill stuff and run out. It would be interesting to actually investigate and get involved since her quest is very closely connected to the main quest.
Vivienne should have had a bigger role in Winter Palace, she probably even could have been a mandatory companion there regardless of whenever you recruit her or not. People should also be allowed to bicker with her more and then lose the argument to her every single time, it would give the opportunity to flesh her character more, make her superiority more believable .
Sera more or less was fine, I know people like to have the option to punch her and other adore her, so I would say that as far as Sera goes "mission accomplished". Speaking about punching, companions are out to be able to punch PC, it should not be one way street where PC can get physical. The dialogue options with her felt a bit campy at time.
Blackwall also has an ok quest, but it also lacks the aftermath part, after so much drama it feels much ado about nothing.
Varric is the most problematic to me as companion, mostly because the lyrium drama is very easy to be dismissed, so his quest doesn't seem all that important.
Dorian has problem in that if you don't romance him, the interactions with him can become non-existent once you do his father quest. It's as if he only exists for that quest and nothing else.
I think Solas is fine as is. I like the fact that much about him is locked in romance (I never romanced him)
Cole is ok too, a bit more interaction would not hurt. His introduction on the Templar side is much better than the mage side, especially if you are not familiar with the books.
I think I didn't missed anyone....