I use @pm.me (Protonmail’s domain) and I’ve had a couple of instances where emails I send bounce or email systems of certain corps won’t deliver to me. It’s pretty rare but it happens maybe once a year or so. Notably the web server of the government of the city I live in (Austin, TX) won’t accept emails from that domain so I have to use my Gmail to communicate with them.

I think the biggest issue is signing up to websites. I support a .org.au domain and get occasional complaints people can't use online services that declare it to not be a valid tld.