All I know is that I have it disabled on my phone except for the couple of services I do want (like Find My), and it works fine. My photos get backed up using Google Photos app, etc
I think if you turn it off entirely, it'll ask you once for each OS update if you want to sign in, which isn't too bad.
When it really screams is if you log into iCloud then forget your password and it keeps asking you to input it. Like with every elderly member of my extended family.
