Un-fucking-believable breaks this.
“Un-fucking-believable” is standard usage, but “unbe-fucking-lievable” still works as an alternate. That’s when you’re down to artistic judgment and choosing which form fits your case best
Unbe-fucking-lievable
Ngl that still works
There’s a morpheme boundary here, probably has something to do with it. The examples in the post have no morpheme boundary before the main stress, or at least not one that’s transparent to English speakers (ab/solu/te/ly might have hypothetically been more transparent to a Latin speaker though)
I’d hypothesise that you can insert fucking either between the prefix and the root, or before a stressed vowel.