Durant would be perfect for the Celtics, and he would thrive.
Howard is a very fine player, but he doesn't look like a fit - well, a team and a player always have to fit to each other, but in his case the team has got to do a lot.
I think that the biggest issue is not so much his weaknesses (FT shooting comes to mind, and turnovers, and the big salary cap hit...) as how he wants to play. He's been pretty explicit about wanting the ball more, and he's been openly dissatisfied about not getting it. He expressed his disappointment to the media about his meeting with Morey on this, which is maybe the point of no return, literally...
If he would accept a DeAndre Jordan-type role, it could perhaps be made to work.