I generally dislike musicals (with some exceptions like Dancer in the Dark and Moulin Rouge!), but the Back to the Future franchise seems like a bad fit for a musical.
More so than most franchises, Back to the Future feels very tied to the zeitgeist of its time (80s to early 90s).
Then again, they made Joker 2 a musical (I recently found out about this and had to double check that this wasn't a joke).
Let's see how those plays out, hopefully it won't be lame.
Btw, if you go in without any expectations, The Fly II is decent. Just don't compare it to the Cronenberg release and take it for what it is.