re: How can one abuse "Unlimited PTO"?

OP, I have experienced this from the other side, as first-line manager of a department whose employees received "unlimited PTO."

Our HR department was adamant about the benefit being real and the "unlimited" description being accurate. When I had people (in my opinion) abusing the situation, I was told that it wasn't really our call to make (referring to me and the middle manager for whom I worked).

A hypothetical example I asked about was "what if an employee uses her 'unlimited PTO' to extend paid maternity leave beyond its usual duration?". Basically, what if someone submits for maternity leave and also for an additional 2-3 weeks of "unlimited PTO" immediately after?

I was told this was 100% kosher and that it wasn't my role to question it.

As a result, I had to be extremely diligent about 1) documenting poor performance when actually working; and 2) enforcing the rules we could enforce, about things like giving me advance notice for PTO.

This all sounds pretty bad for a manager, and it was, but frankly it was no worse than the issues created by old-timers with lots of PTO blocking off big vacations months in advance. That's equally infuriating for someone trying to meet deadlines.