I've worked at a small, mid-sized and large company, and unlimited time off worked the same at all three places.
Unless it is an emergency, communicate early and often, and it is not a problem. That way, you and everyone who depends on you knows what to expect (or not) and when to expect it (or not).
The best part about it is not worrying over small details, like if I take my time off in the summer and fall, will I run out of PTO before flu season hits this winter?
Yes, it is unfortunate that you dont have anything to cash out once you quit, and you might worry about taking too much off. As to the first, oh well, take the time off. As to the second, if you can't have an honest and fair conversation with you boss about your concerns regarding time off, you aren't going to have honest or fair conversations about a lot of really important work matters that process and time-tracking and firm clear cut rules won't fix.
Expect to be treated like an adult, act like an adult, and if you aren't treated like one, take advantage of the good market now and find a better job. That advice applies to a lot of maybe-maybe worries in the workplace for in-demand positions.