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”)

Part of the reason I prefer having a catch-all on my own domain is that I can change providers without changing any email addresses. For example at the moment I run my own server, but in the future if that becomes too time consuming I can easily start paying for a service.

ETA: also I’ve never gotten any spam to a email I haven’t given out, people don’t really send emails to random names at a domain as far as I can tell