The biggest issue for me is that Apple are adamant that @me.com and @icloud.com are interchangeable, and they aren't.
You can sign your AppleID in to an iPhone or MacOS with either suffix, but if you created the account during the @me.com period only using @me.com works correctly.
I've spent countless days with Apple Support, including direct "engineering assistance" and while they are absolute in their statements that you can use either, you can't.
My iCloud Music Library was fundamentally broken for weeks, no amount of deleting, reformatting, reinstalling would fix it. Eventually signing in with @me.com not @icloud.com fixed it. Apple never agreed.
Then when my kids got iPhones, Screen Time features through family sharing (me monitoring my kids Screen Time) would not work. Removing my kids from my family account (they were linked by @me.com) and adding them back as @icloud.com fixed it. In one of the AppleID pages online you can see which is your true AppleID domain.
I demoed the fix by removing and then adding my teenagers with the other suffix and Screen Time worked, but they refused to believe it would fix the younger child. I couldn't fix it myself as he was under 13 and so can't be removed from a family without being associated with a new one, and you can't then add them back until the end of a waiting period.
Eventually I had Apple Support "correct" his DOB to make him 12 years and 364 days old (they wouldn't make him a teenager). Then I waited 24 hours and could remove and rejoin the account from the family and magically it was fixed.
The closer they get to shutting down all references to the non-iCloud accounts, the less chance they'll ever fix these bugs!