Unlimited vacation as a perk at US tech companies and startups in particular is a total illusion. I'm sure they never use 4 weeks a year that most Europeans are legally entitled to and if they tried would probably find that it is not nearly as "unlimited" as advertised.

I don't know why you're getting downvoted, it's exactly true. In EU if you have 28 days of paid leave, the company has to give you that time off, no exceptions. In US companies with "unlimited" vacation time it's not really unlimited because most places would really not like if you took 28 days off a year(and promptly exercise their right to at will employment) and a lot of companies would say ok, you can have unlimited leave but you have to be on call. In Germany it's literally illegal for employees to bother employees while they are on holiday.