Moved from ProtonMail to iCloud+ last year since I was already paying for iCloud+. Proton’s lack of IMAP support on mobile devices is another reason I left. Their reason for that is privacy, which I understand. Privacy is important to me, but this is email we’re talking about. It will never be private considering the majority of people we email use Gmail, Outlook, etc.
All that being said, iCloud+ has been solid for email. No major issues at all on my end. The spam filter is quite strict for me, but I would consider that a good thing.
Same here, iCloud+ works great for me, the domain setup was painless and you can even create multiple aliases & inboxes and share the domain with your iCloud Family if you have one.