This is a huge selling point. Private email relays often get blocked, the only lasting solution is to put them on the same domain as, and in the same format as, a significant number of non-private email addresses. As far as I’m aware the only other provider doing this is Fastmail.

Comments about lock-in aren’t wrong, but it has to be this way. You can make arbitrary email addresses at your own domain, but anybody who feels like it can trivially automatically detect that those are all you.

Personally I use unique at own domain only where I’m identifying myself anyway, like my bank, and Fastmail masked email where I’m not. For most things it’s not actually that terrible to accept a small risk that they’re offline for a day between your being booted without warning and you changing your email address with them.