Two slightly different outcomes at my last "unlimited pto" workplace was like:

1. Lady right before my time took unlimited PTO off for some medical stuff. Extended it when it turned out to be serious. Was able to get paid while out. Retired after like 2 months out. Unfortunately I think it was cancer so I think she passed. (this was like 3 years ago, I think we were in 1 or 2 calls at the same time. And this was tail-end of covid. So I didn't know her or keep up with it much.) But at least was able to get time off and paid for it without question.

2. I decided that I was done it + other reasons, so I started "quiet quitting". They finally caught on after like 3 months of me half assing my work. I probably could have asked for time off but really did not want to push it and them decide to sue me for "stealing" time off or some shit. So I just walked out one day basically. I almost certain made up with 'unofficial' PTO but probably also should have been owed more.

It sucked because it was completely up to you to decided how much time off you should ask for. Like they said they would allocate you an appropriate number of stories (aka hours in software dev). But really the planning didn't matter. It really screwed me up because it meant that if my ADHD and depression having ass mis-estimated it, it really set up a disconnect between me and my boss. I could have handled it (and did for like 3 years) but decided I didn't want to get one more task from a mil-industrial company that suddenly was in our roster.

Their system was definitely "good" because if you kept up on appearances (did all your assigned tasks + 10%), you could pretty easily take off like a month in summer + 3-4 weeks around xmas (virtually everyone did so no one cared if you were out too). I imagine ANYONE really struggling to keep up would have been screwed out of so much time off.