Compared to simplelogin (or proton pass aliases, addy, firefox relay, etc), one other downside of a catchall is in associations across accounts. Registering with a @passmail.net address implies that I use Proton; registering with random-string@mydomain.org implies I have access to that domain. If 10 data breach leaks have exactly one account matching the latter pattern then that's a strong sign the domain isn't shared. If one breached site has my mailing address, my real identity can be tied to all the others.