it's pointless to self-host mail (except for backups maybe) because 99% of your normal-life recipients are likely to be on Google and the content of your emails will be ingested by the big brother anyway.
The reason I do it is that E-mail is often the only identity on other services I use (it’s often the only way to reset your passwords), and I feel it is too risky to be dependent on a megacorp for my identity, a megacorp who may decide out of the blue to suspend my account for no reason.
I’m dependent on my VPS provider, but I have backups and can take my Debian config with me to a different one. Or bare metal if I have to.
I’m dependent on my DNS registrar and haven’t figured out a good way to mitigate that risk.
Besides that, that’s it.