I think they're competent in their craft, but beholden to too many restrictions and people above them enforcing their restrictions.
You got the story that needs telling
The story that the game is telling
The story how the playerbase is experiencing it (not the same)
The knowledge of the next expanion roughly shaped they cant undermine through their writing
The plans Blizzard has for current and future characters
etc etc etc.
Lack of creative freedom creates awkward writing. The fact that a bunch of stories were still entertaining is more a testament to their ability than anything else.