Based on my experience running mail servers for a long time and today, I'd say OP is right.
gmail spam filtering is terrible. On my various gmail accounts, I both get spam and the good email goes to the spam folder. And there's nothing you can do, you can mark it not-spam a thousand times and it's still a crapshoot.
I don't have any of those problems with my self-hosted email.
Gmail spam filtering is top notch. I just stopped to care to obfuscate or hide my email adress (which I use since the beta invitation program of gmail) and I can count the spam I actually read in a year with one hand.
It's easy to bring down the number of false negatives if you allow the number of false positives to be arbitrarily large.
On my GSuite business email, I've had > 50 incoming business-relevant emails this year that were incorrectly classified as spam. My personal self-hosted email server [1] lets through a bit more spam than Gmail, but it also doesn't suffer this big false-positive rate.
"I can count the spam I actually read in a year with one hand."
This is partly because Gmail is good at classifying emails as spam/ham.
But it's partly because it's more tolerant of false positives (ham sent to the spam folder) than you or I would be if we were tweaking our own spam filter.
I occasionally check my spam folder, and there are usually some mailing list emails that I don't care about, but which I did actually subscribe to, and would have wanted to reach my inbox.
gmail spam filtering is terrible. On my various gmail accounts, I both get spam and the good email goes to the spam folder. And there's nothing you can do, you can mark it not-spam a thousand times and it's still a crapshoot.
I don't have any of those problems with my self-hosted email.