I used to run a hybrid mobile app + webapp company.

Private emails regularly lead to awful customer service interactions because people cannot tell us the email they used to register. Fastmail at least is off the beaten path enough that people probably can understand. Apple, especially using sign in with Apple, is horrid. And not just people unable to tell us the email; they then create multiple accounts; try to sign in on web and use their actual email and then have 2 accounts and flip shit that their stuff is gone; etc. Oh, and regularly blame us for their confusion.