I don't think it's "lot of" really, must likely few or maybe even none, but frankly i won't test it, i'm just quite sure that jass include many internal safety checks for native functions.
Hell, instead of some constants you can even just use 'null' instead.
I don't know about double free, i suppose you're right, i've never really "used" them.
Anf ofc it will crash the thread if you use a not initialized variable, but that's not the same thing.