[drudoo]

I always sign up directly and store the information in 1password. With a custom domain I always use <website>@myDomain.com.

What if your twitter/fb/google account gets suspended for whatever reason? All of a sudden you can’t login to a plethora of sites.

[lordofmoria]

Yes, but that's more of a general argument against all SSO - besides, does your concern hold even for things like Sign In with Google / Apple? I assume losing access to iCloud / Gmail (if you've opted into these ecosystems) is an existential threat anyway, and SSO is the least of your worries at that point, and the question still stands.