1) It is a lie. It is certainly NOT "unlimited" in any way, shape or form, and everyone knows it. Can you take 10 years off? How about 1 year off? How about 6 months off? Nope. "Unlimited" is a lie. There is a limit. But now it is a "secret" limit.
2) Unlimited PTO = Limited PTO and the limit is 100% determined by your manager. If you manager likes you, your secret limit will be high. If you manager does not like you, your limit will be low or even ZERO. Unlimited PTO means a manager can just refuse to approve any of your requests and there is NOTHING you can do about it.
3) Unlimited PTO means extra confusion for everyone. Instead of knowing that you can count on 2, 3 or 4 weeks vacation, you simply never know how much time you can get. And this time can change at ANY time.
4) Unlimited PTO means unequal time off for people. Some people will get to take more time off than others. It is guaranteed to be unfair and guaranteed to be gamed. Basically, it encourages people to take time off early for every excuse (instead of saving time up). Also, it encourages lying. You want to take some time off, better to claim that your kid is sick than to be honest and say you want to go to a baseball game.
5) Unlimited PTO = no privacy. Now you have to explain and justify every time you want to take a few days off. When you have 2, 3 or 4 guaranteed weeks off, you just say "I wish to take my X weeks off at these times". You do not need to explain that you going to just lay in front of the TV and binge watch 90's shows. But with Unlimited PTO, there are no guarantees. So now your manager can demand that you justify every request. Instead of notifying, now you have to beg.
6) Unlimited time off is am employee-hostile position. Imagine if a company offered a "unlimited salary". You get paid whatever your boss wants to pay you every month. It might be zero or it might be a million bucks. Would you agree to that? No. No one would. Then WHY would you agree to an unlimited PTO?