Since deleting my Google account early this year (having used it as a primary account for e-mail for about 15 years), I have only stumbled upon two problems. The first one was that an old account I had with my current ISP had tied my phone number and my old e-mail account. This was solved with a call to customer service that severed that link and allowed me to make a new account with my phone number. The second was that I missed an invitation to an alumni party from a previous job... so yeah...

Note, I did spend a significant time overlapping my new primary mail account with the Gmail-account to ensure I had resolved as good as all of these issues. Since moving to Proton, I think I spent 2 years before actually deleting my Google account. That included a fairly thorough cleanup of old accounts that took quite some time and energy (boy, do some of these services give you a hard time deleting old accounts....)