Therein lies the problem… Define “abuse it.”? If there is such a thing than PTO should be limited to that ;) Dont say it is unlimited and then say that actually is limited…
If the workplace culture isn't such that it is clear what abuse of the policy is, then yeah, it's not going to work out. Expectations have to be clear.
I think the best way to define what abuse is, is give explicit numbers. Even something like "25 days/year free use PTO, 20 more days to be discussed with your manager" or whatever. I came up with that policy right now, I haven't seen it anywhere, but point is - setting expectations beforehand is much better than just saying "unlimited". For some, unlimited means "20 days if you're lucky", for others it's "up to 40 days we don't bat an eye, if your output is good otherwise". "Unlimited" doesn't convey that. I want to know before I start working. Please convey it.
If the workplace culture isn't such that it is clear what abuse of the policy is, then yeah, it's not going to work out. Expectations have to be clear.