I don't understand unlimited PTO. I mean, why not just take every Friday off, then? There is almost never a point at where where there's no work to be done; it's never going to be a case of "as long as you get your work done, you can take time off"... because there's no such thing as "done", just "prioritized".
If you can be competitive with peers and get enough done to continue progressing in your career with every Friday off, why not? The limit for me has always been my own productivity (and availability for meetings), and I think at a certain role level it's a reasonable expectation that you be measured in outcomes and not time spent at desk.
There are still expectations on the amount or level of work to be done. If you're meeting those and not blocking others, there's no reason one couldn't take more time off.
It's certainly a tricky thing to sort out, though. As you noted, "abuse" is possible, but defining what constitutes abuse is nearly impossible.
