[gumptionary]

As others have said, unlimited PTO ultimately means that it's up to the manager to determine if you can take days off. This places 100% of the burden on the manager to be the 'bad guy' who has to say no to a request, thus building in an unnecessary opportunity for conflict into that relationship. There's also a burden on the employee making the request to somehow know if they've taken the right amount of vacation.

PTO is one of those things where, to misuse the quote, 'good fences make good neighbors.' A reasonable set of requirements and limits can reduce the guesswork, preserve relationships, and make it so people actually use their PTO.