The problem is to my boss "unlimited" might mean 15 days and to me it might mean 4 month. If we come in with wildly different ideas about what is OK, we're both going to end up very unhappy.
Or you could have a straightforward discussion with him/her about your expectations.
The real issue with unlimited PTO is that many people are so conflict-adverse (except on the internet) that they cannot initiate basic conversations about how much vacation time they think is reasonable. And instead of taking responsibility for communicating their expectations like adults, they stew in resentment against their employer for shifting that responsibility onto them.
If your employer balks at your idea of reasonable time off, then you might think about switching employers. But if you balk at the idea of having some input into your employer's expectations of you, then you need to take more responsibility, or move to a company with a less collaborative culture.
Why are you positioning this like it's the employee's fault? I've had a gig with "unlimited" vacation time where my boss was very quietly stewing that he thought I was taking too much time. Never said anything to me until two weeks before I left.
It is in all cases management's job to set these expectations, and their fault when they're not understood on both ends of things. That's why they're management. (And why "unlimited" policies are stupid.)
The whole reason you have policy and leadership is to handle these kinds of situations though. People are conflict averse, especially when they are not in a position of power over the person they need to talk with.
