“If the vacation is non-accrual then it can also mean you don’t get any vacation at all if they say too bad. In addition should you quit you don’t get to cash out your vacation since it isn’t accrued.”
I think you hit the nail on the head here.
I’d check their holiday calendar. In theory flexible PTO could have advantages with the right manager; it does beat having to burn PTO to go to the Dentist or stay home with a sick kid. But I like the fact that because I rarely take vacations I have two weeks accumulated that I could cash out if I left or got downsized.