One of the features seems to be a "hide my email" feature, akin to Apple's hide my email or Fastmail's masked email feature.

Having used both of those, I would say one downside is that occasionally, a site will detect that I used the Apple one, which is strange because it's just an iCloud email address. Perhaps they're looking for a specific pattern.

I haven't yet seen the Fastmail one blocked.

One concern with the Proton one is that it seems like its masked emails are all at passmail.com. I've already found some sites block protonmail, so they'll surely block passmail like they do Mailinator and other sites. That could be a limitation that's less likely to affect Fastmail's service.