Migrating DNS providers is a pain - recently done it twice. Transfer itself is reasonable with most providers. Importing/exporting a BIND-formatted zone file is sometimes unheard of, as is setting custom TTL; you'll have to go through a stupid form. One provider tries to hold your hand so tightly it won't let you set CAA with iodef, only issue/issuewild.
Migrating email is a pain. Yes! You can just point your MX elsewhere, and that is brilliant. You still want to copy over all your email, and given IMAP has won, if you don't have a recent backup (who does back up their email?), losing your old account sucks.
Fixing up your email clients is also troublesome. You can't just CNAME smtp.yourdomain.com to smtp.example.com, because that's nuts, so changing providers from example.com to beispiel.de requires a couple more dances; provider docs also suck, and email clients usually fail a dozen times before you can find the right incantation. You could set up your own autodiscover, but that requires an HTTPS server.
Yes there are providers that sell a full package and do all the initial setup for you, but that's not the point of owning your domain.
Yeah, I sometimes do sysadmin stuff for fun. None of this is fun.