I've been using Privacy.com cards for a handful of years now with very few issues. I think a card got rejected only once. The funny thing is that when I used that vendor-locked card again on that site, it worked just fine. I was really impressed when a real human helped me set up my account when my credit union was not in their list. They made it really easy.
I really love using this service for recurring or auto-renewing payments. My ISP keeps raising fees without much (if any) notice so my card gets declined when they do so because it is over the set spending limit for that month. That allows me to assess whether or not I want to try to find an alternative. Unfortunately, with ISPs, they are pretty much oligopolies so I get to pick from bad option 1 and bad option 2. But for other cases, it helps me keep a handle on the enshittification of services. And when they force you into auto-renew, just pause the card and you can control whether or not you want to renew a service.
