Part of the problem with unlimited vacation is that companies that use it tend to do it to save money, since if you have unlimited vacation days they don't need to compensate you for unused PTO when you leave. They'll also try and create a culture where employees don't want to or "can't" take time off. It can be more nefarious than anything.
Of course companies that do that are usually startups, and if your startup tries to fuck you, leave. They probably need you more than you need them.
The financial aspect is certainly true though, as someone who hasn't moved around a lot and who pretty much uses all their vacation, that aspect of traditional vacation banking has never been a big deal to me.
The culture aspect needs to come down from the top. A senior engineering manager at a well-known SV company with "unlimited" vacation claims it works pretty well because, from his perspective, an expectation that you'll take time off and disconnect comes from the top. On the other hand, I've heard others at the same company give a less rosy report, so YMMV depending on teams, etc.
People will find ways to get the time and rest they need, hiding and faking is less effective.
On top of that you get loyalty in return for being treated as human beings rather than slaves.