The UAW has two large issues:
1) If they treat Tesla operations like it is another auto manufacturers, which is quite possible because that's what they know, there would be a huge culture clash. And clashing with Musk isn't a good, productive place to be because he's hardly the easiest going guy around.
I don't know if UAW is willing to learn and grow? On Tesla's side they'd do well to insulate the whole thing from Musk himself with someone Musk trusts.
2) A much bigger issue is UAW's conflict of interest. They have far more members at other manufacturers, and Tesla represents an existential threat to those manufactures and therefore to the jobs of those UAW's members. Further, Tesla is aiming to reduce assembly work on the vehicle by about 10% (I think that's right?). Now that doesn't mean that UAW
will act in bad faith but that doesn't fix a conflict of interest. Just the appearance of a conflict of interest is bad enough, it poisons the situation.