There isn't much breaking usually, I really don't remember big issues in my 15+ years self-hosting time. I have changed colocation place several times, some places have been offices with average office internet (without redundant links or BGP peering).

I can see issues, if you over-engineer and try build some microservice farm in some cloud provider, but simple physical server with DC grade disks in RAID and backup (tested and out of server) is pretty reliable.

Of course, when you really don't want to do it, then paying for someone else is reasonable, no issue with that. But its not fair to make mail self-hosting look like something very complicated and dangerous, I would say that modern web service stacks are more complicated and fragile. Lots of guys here are writing own internet facing software, handling customer data. Compared to that, using pre-existing mail server software isn't that hard.

its not always about saving a buck. i spend more in hosting +domain than i would if i went with fastmail or zoho or whatever. what it feels like is going from google to zoho. when you own the server, you own the backup on s3, you know you can just switch servers if need be. your addresses and everything comes with you.

i see this as a hobby if nothing else. maybe others have the same idea