There is another sort of worst case (company's perspective): An employee thinks they need to rebuild their house and needs to do it all by hand, hence will be on leave for the next six months. Or, say, wants to explore Europe backpacking and hence needs break for the next two months.

With unlimited PTO the biggest challenge is to define (both ways) what qualifies for a good reason to go on a leave.

I have enjoyed unlimited PTO wherever I had. But I tried to have my own benchmark of about four weeks in a year. Of course, there have been times when I needed more, and it was fine. There have been times when I didn't need four weeks either, and I was okay with that too!