This post is pretty horrifying. If you're in the Apple ecosystem, Apple Mail and iCloud Mail (standards-based, supports custom domains) has been as reliable as any mail provider and client I've ever used.
Eventually our subscribers will get in touch with us asking why our emails are no longer getting sent to them. This is with very low spam rates, DKIM, and DMARC set up. There's also no good way to contact the iCloud postmaster, which is an option for every other major email service.
I also have had several instances where iCloud Drive would take forever to sync. The most recent time got so bad (100% cpu usage that persists after killing the process when I added 5KB worth of files) that I stopped using it completely. Tried Microsoft OneDrive instead and it synced at good speeds and gave me no problems.