When I joined the company one developer had no vacation for more than eight months. She was Russian and was relocated to a country where she had no relationship... si she just worked. That was a huge liability until she took one week off to interview and left then company one month later.
The problem I've found is that you don't really know which you're gonna get. What's the limit is a secret few companies openly speak about to leverage the fact that generally speaking people take less PTO with unlimited PTO. Some of the companies I've been at combat this by having vacation minimums or official office closed days or having leadership leas by example, but it's honestly a hard problem to solve because a surprising amount of people will not take vacation without the pressure of use it or lose it because they are natural workaholics.
I have also found that people are generally more stricter about how good your performance has to be the more vacation you take. I've found that despite taking like 6 weeks (which is crazy for an American), most companies have been cool with it because I perform well, but people who take 3-4 weeks get way more pressure for average performance. Is suspect this bias is also why some people don't take off.
We don't technically have unlimited PTO at my workplace, but this is how it works. The only thing the PTO cap affects is whether or not you're going to be paid for the time off, not whether or not you can take the time off.
There are pros and cons to unlimited PTO policies for both employer and employee. It's almost always promoted as an attractive perk in job postings, but I would much rather have the clarity of a fixed amount of paid leave, so long as it's a reasonable amount.
I would like to see organizations being more creative about PTO. For example, haven't there been studies that have shown that 4-day weeks lead to increased productivity?
As for 4 day work weeks. That's always a special topic for me because while it's great for certain kinds of workers, it's just materially bad for a lot of others. And then there's the elephant in the room that shows that literally every school district that moved to 4 day weeks has worse outcomes for children. Their grades are lower and health outcomes are worse because schools act as a safe harbor for underprivileged children.
I do know plenty of companies that play with schedules, but I've seen people tend towards 5 8 hour days. The only thing I've seen have success is flex hours, vacation minimums (which is really just accrued under a different name), and forced time off usually in December or in the summer if have a natural workaholic staff.