Not GP, but it matches my experience: with reverse DNS, DKIM, SPF, being in DNSWL (and not in DNSBLs) some of my messages end up in Gmail spam folders (very recently received a reply to a message I sent 3 years ago because of that, for instance). A while ago there were strange issues with Google-hosted mailing lists as well, with some messages not being visible even to an ML administrator, while Gmail servers accepted mail just fine (maybe it still happens, I'm just not writing to those). I hear similar stories from other mail server administrators too.
I think it's more of a Gmail/BigMailerCorp issue, and still using a private mail server myself, but I don't think it's fair to say that it's all easy and works flawlessly, even with strangely behaving mail servers on the other end.
I recently checked my Trash folder in Gmail. I had emails in there that were several years old, right below the text that says Gmail deletes emails in the Trash folder older than 30 days.
Indeed, a quick search suggests that spam is automatically removed every 30 days there. The wording was "Google hid it from me", so either I have wrongly assumed that it's about the spam folder, or it is possible to disable autoremoval.
Edit: or, as the sibling comment suggests, it behaves strangely with respect to removal as well.
Or user accidentally archived it. “X did Y” is often a user’s way of saying “I accidentally made X do Y” without having to accept blame. It’s the “dog ate my homework” of excuses.
I think it's more of a Gmail/BigMailerCorp issue, and still using a private mail server myself, but I don't think it's fair to say that it's all easy and works flawlessly, even with strangely behaving mail servers on the other end.