Custom domains mean that if the alias provider enshittifies, you can switch to any other provider near-instantly. As long as you never use the domains to host illegal or dodgy shit it’s extremely unlikely you’ll ever lose them — far less likely than losing a gmail or whatever.

With SL you can avoid spam by using the “beta” (been beta for 3+ years lol) “auto create” option instead of a catch-all, meaning that you can direct emails to different inboxes (or do nothing) based on specific regex strings you control — up to 100 of them. I had a catch-all regex (.*) as my # 100 and it took 2 years to receive catch-all fishing spam. Then I removed it and now have only random strings (e.g. .*fgyu.*) so new emails must have them if they want to get somewhere. Everything else bounces. All previous emails continue to work until you disable them individually.

I use a mix:

• SL-domains: anything I don’t give a shit about.
• Non-PII domain: anything I would want to persist if I changed provider, but don’t need my identity, or can give out a unique email in-person.
• PII-domain: banks and all other services tied to my identity.
• Top-Secret-PII-domain: critical services that could compromise all others (password manager, email/OS accounts, domain name registrar).

I have basically the same thoughts as you. The reasons I can think of is:

• Convenience (but SL is pretty convenient)
• Less of a lock-in to one vendor.
• Avoiding filters on sites not allowing aliasing domains (often incorrectly under the label “temporary email addresses”)

Both are fine choices depending on your requirements. The thing with external alias services, you are not in control of the addresses/domain. Catch-all addresses are essentially aliases you manage, but something like Simplelogin does have the benefit of hiding your domain name.

Spam is not a big deal on catch-all. A couple of times a year I do get a spam mail to some arbirtary address, but that’s more or less it.