I viewed this as a red flag too until I landed at a company where the unlimited vacation policy was sincere. It was actually a great experience.
At risk of sounding like a management consultant, it was very much a matter of corporate culture. People at upper levels took vacation regularly and actively encouraged their reports to do so. One-on-ones often included asking about recent and planned time off, and "none, none" produced real pressure to use the policy. The lack of set days did save money when people left, which probably helped it stick around, but I think the initial adoption of 'unlimited' instead of some generous allowance was basically just a matter of not wanting to formally track vacation time. (One other significant point, especially for tech: there wasn't an expectation of having grand plans, and people who weren't taking time off would even be reminded that they didn't need any "good reason" to do so. I've definitely heard of places where taking a month off to hike the Andes is fine, but taking two days to catch up on sleep and errands is frowned on.)
I still don't take "unlimited vacation" seriously when I see companies advertise it; I'd want an account of the real policy, minimally from Glassdoor reviews or preferably from an employee I knew. But I almost never see US companies advertise 20+ days of paid vacation as a starting allowance, so if the goal is to maximize time off I suspect searching for 'real' unlimited might still be the best bet.
At risk of sounding like a management consultant, it was very much a matter of corporate culture. People at upper levels took vacation regularly and actively encouraged their reports to do so. One-on-ones often included asking about recent and planned time off, and "none, none" produced real pressure to use the policy. The lack of set days did save money when people left, which probably helped it stick around, but I think the initial adoption of 'unlimited' instead of some generous allowance was basically just a matter of not wanting to formally track vacation time. (One other significant point, especially for tech: there wasn't an expectation of having grand plans, and people who weren't taking time off would even be reminded that they didn't need any "good reason" to do so. I've definitely heard of places where taking a month off to hike the Andes is fine, but taking two days to catch up on sleep and errands is frowned on.)
I still don't take "unlimited vacation" seriously when I see companies advertise it; I'd want an account of the real policy, minimally from Glassdoor reviews or preferably from an employee I knew. But I almost never see US companies advertise 20+ days of paid vacation as a starting allowance, so if the goal is to maximize time off I suspect searching for 'real' unlimited might still be the best bet.