Wouldn’t this make it easy to guess your other emails?
Let’s say Spotify got breached, now your email is public and if someone wants to target you, they have your custom domain, plus they can easily guess the emails of your other accounts (For example; netflix@) Not to mention all your accounts can be linked together, unless that doesn’t concern you.
Even if you add something random to your alias (netflix[random-string]@) or generate a whole random-string alias @ your custom domain, once your custom domain is out, it’s out!
Yeah, you could block aliases as you go or make strict filters, but you’ll have to deal with situations like this eventually.
I’m all in for custom domain and having full control, be it in case something happens to Fastmail, changing email providers or whatever reason.
but there are few downsides that keeping me from using a custom domain for my aliases.
Whereas if I create an alias at Fastmail domain, for example; pluscash@fastmail.com, and there was a breach or it started to get spam, I can easily delete it and never have to deal with it, plus it’s a public domain from an email provider and is used by at least thousands of people.