Them being balanced or better than each other has nothing to do with compatibility.Yeah. We have different definitions. I don't agree that LevelUp is compatible. Having looked at the playtest documents and some stuff since, I wouldn't allow playtest classes to play along side 5e classes. The LevelUp versions are better. To use an earlier term, they don't play well together. The differences are greater than those of 5.5e vs. 5e.
I would and do use monsters from my monstrous menagerie book in my 5e game, though. They're simply updated, better versions of monsters, similar to ones that are homebrewed by DMs.
Someday I hope to convince my group to make the switch to LevelUp, but until then monsters will remain the only thing that makes it into the game.