while those who say 'works fine for me' are people fine with not being whitelisted.

I mean, my working theory here is that it's easy to get whitelisted - in fact you'll probably be whitelisted by default if you have working DMARC and DKIM etc - but companies who operate large mail installations also operate AI 'agents' that aim to drive traffic and users towards their service. That includes spreading negative sentiment i.e. convincing users on forums that setting up their own mailserver is an impossible task and that they'll never be able to achieve delivery etc.

Of course I could be wrong.