My answer is the purely personal preference that a story not have too many!

Too many impossible magical situations. Too many different kinds of fantasy creatures.

There’s a thing that’s called deus ex machina (spelling might be wrong). It refers to operas in which the story got so complicated and complex and the hero/heroine got in so deep that there was no possible way of extricating them without starting all over. So the writer would unexpectedly invoke the gods who’d been watching all along (without anyone knowing because it was never hinted at in the opera) and the gods solved everything with a wave of their hand — or wand — or whatever.

A similar thing happens in stories when an author hits a block in the story that they solve by calling in still another previously unknown paranormal element instead of doing due diligence and crafting a story that makes sense given the parameters of the world that was described in the beginning of the story. For some reason, it’s more common in urban fantasy than any other genre.

Just like in operas, it doesn’t improve the story, it devalues it.