There are two major ramifications of unlimited vacation policies:
1) It changes how you feel about taking time off. Rather than paid time off being something that you have "earned", you evaluate your PTO in comparison to your teammates - specifically most people try to make sure they are not taking off more than the average. The end result being everyone is subconsciously competing with everyone else to consume less PTO than average.
2) Since you have not accrued vacation time, your company does not owe you financial compensation for vacation time not spent when you leave. As someone that changes jobs every 3 or 4 years, this is more painful than it sounds.
If I ever decide to be in a position that affords me the opportunity, I think I would enact a "minimum vacation policy". It's still unlimited, for whatever that's worth, but employees will be required to take a minimum amount of vacation every year.