So I think I figured out what Fifteen's problem is. Not as a flaw in the writing, mind. A flaw in the character.
Every New Who Doctor has been a reaction to the trauma of the Time War.
Nine raged
Ten ran
Eleven repressed
Twelve wallowed
Thirteen regressed
Fourteen finally admitted he had a problem and sought help.
And Fifteen, I'm certain now, relapses.
Fifteen started out doing great. All his trauma processed. Ready to face the world. And maybe at first it worked (though sadly we didn't get to see his many offscreen adventures with Ruby). But like... you don't stay better. You have to work at it. And he's very much not doing that.
There's a brittleness to his good cheer. Be it the gods of chaos, the loss of a lover, or his first real confrontation with racism, he doesn't deal with adversity very well. He breaks down, he hides, he acts like everything's fine. It's not fine though. He's not okay. He might've dealt with his problems, but he didn't equip himself with the tools needed to deal with new problems.
He's gonna crack eventually. We can already see it happening.
I've been thinking about it an unhealthy amount... But like we have sooo much dark, gritty sci-fi now. Black Mirror just came out with another season last week that shows us the darkest sides of the future. Paid advertising in medicine etc etc.
Why shouldn't RTD let an alien twink hoover up incels with a retro-future Roomba? Or fight a fifty foot fifties cartoon character. It's a universe where literally anything can happen and I think that's awesome.
It's the goofiness that sets Who apart and a lot of people forgot that after Moffat's run.