If you are not self-hosting the simplelogin stack than you have to trust them just like with every email provider. It’s a downside of e-mail as a protocol, that it is not easy to self host and the provider gets your plain text mails.

If you can trust simplelogin than you gain a lot of advantages because you never have to give out your real email address.

If a burner address gets leaked you can simply disable it and know which site leaked it because you use a different address for every site.

Also you cannot be tracked across sites by your email address as a unique identifier.