Unlimited PTO is one of those ideas that sounds amazing on paper, that doesn't tend to translate as well to how people act.
1.) Studies show that people tend to actually take less time off, than if they accrue a set amount over an amount of time. People tend to be worried about abusing it.
2.) You also lose the ability to accrue and get a pay out when you leave.
A lot of this is based on the environment, stuff like who your peers are and who your boss is.
One potential fix to that, is to mandate a minimum number of days. You have unlimited days off, but you have to at least take 10 for example.