I think Garris is not very suited for horror or suspense. There is a real lack of tension in his movies, they go through the motions but don't create an overall sense of dread, fear or urgency. Something is missing, I don't feel for the characters, even when they're good actors.
However I think he made the most of The Stand, that is probably not easy to do for tv and on the budget he had. But The Stand has a different tone, it is more dark fantasy than purely horror, so maybe he has a better feel for that - it takes different sensibilities, it has a different kind of scope (more apocalyptic). I think he might be more a sci-fi or fantasy director than horror.
I think he's made more adaptations than other directors, because he's probably easier at going with the demands of the networks. I think a lot of directors have a certain vision and if they can't have that, they quit. I think Garris just changes things according to the demands and proceeds. However since the response to a lot of the films has been negative (at least from many Kingfans and those are most interested in the adaptations), I think he might have called it quits on King adaptations in the meantime - there hasn't been anything since Bag of Bones. Filmprojects are not easy to get off the ground, so negative reception from press and public must always be disappointing.