I had to give up trying to use iCloud for email. So many inbound emails would be silently dropped. I've also sent emails to @icloud.com addresses that the recipient never received.
The deliverability issues also apply to their Hide My Email feature. I frequently miss confirmation or verification emails after signing up with a @privaterelay.appleid.com address, so much so that I don't even bother with it anymore.
We send OTP codes for our login flow and iCloud is definitely a big source of delayed email complaints. Codes eventually arrive, but not before a support ticket is created. Instant on every other ISP.
The deliverability issues also apply to their Hide My Email feature. I frequently miss confirmation or verification emails after signing up with a @privaterelay.appleid.com address, so much so that I don't even bother with it anymore.