I also don't like it and I believe it's not necessarily legal in all countries. Many European countries have a minimum amount of vacation days and companies need to make sure employees also take those (not all do but theres no enforcement unless someone complains). "Unlimited" without a floor might lead to more vacation days for some but likely to fewer days for most.

And if they were truly unlimited, why would anyone work part-time? You could just work full time and take 2 days off per week (plus vacations) at full salary. But I doubt many companies would accept that.