I've been at places where a switch to "unlimited" would basically mean : You no longer have a number you can point to and say "I'm owed this many days of vacation time."

Fortunately, at my current company, when we switched to "unlimited" there was an added "and you're expected to take a minimum of 3 weeks per year". So, while the upper bound is still completely nebulous, by setting a reasonable minimum, it's clear they're not just trying to screw anyone over.