The problem is that "unlimited vacation" is (obviously) not actually unlimited. All it really means is that you can never be quite sure how much vacation you are entitled to. It will of course depend on the company, but as I understand it, "unlimited vacation" rarely actually means that you get even as much as is legally mandated by most European countries (4-6 weeks a year).

If you leave a normal job, you'd be paid your accrued vacation leave. In "unlimited" setups, this gets murky, and normally turns into two weeks. Without a decent ledger, there becomes a stigma to actually using the time. When you "own" that time, it removes the anxiety for both parties.

If you have a fixed PTO allocation, in most states and circumstances, you literally "own" that time. If you are fired, that balance must be paid out. In most states, if you quit, that balance must be paid out.

Like GP, I'm strongly opposed to "unlimited PTO" policies. For the few devs and workers that are helped by it, I think there are too many cases where the ambiguity and uncertainty makes people take less PTO than under a fairly generous, spelled-out policy (and there is usually no accrued balance to be paid out if you leave having taken less than the normal amount).

I won't go quite so far as to say I'd quit if my company made the transition, but it is a strong negative if that is on the benefits docket.

I don't think there's a good way to implement it, unless it's actually unlimited, i.e. you can stay at home for two years while getting paid and it's fine.

If that's not fine, then it's not actually unlimited, and you have to guess where the limit is - so you can't really be guaranteed a certain number of days off.

Though I guess an org could still set a minimum and then call it unlimited, though in practice I expect that that minimum is basically also the maximum.