Has any company with "unlimited" but tracked vacation actually revealed how many days their staff take? And what disparities there are between individuals and departments?

I'd bet that it's overall _less_ time than companies where vacation is metered, but compulsory. e.g. mine in the UK is 25 to 32 days depending on time at the company, plus bank holidays, and you've got to take it.

Peer pressure must conspire to _reduce_ what people take, especially in companies that do "360 degree" peer reviews. And it would only take a hint of management pressure to make staff feel guilty about booking holidays (or cancelling those that they'd already booked).

For me "unlimited holidays" translates to "unlimited work hours", and I'd be suspicious of companies that promote it as a perk.