The difference is night and day. I can look in the spam folder for my work email now and there are hundreds of spam messages there for the last 30 days, and absolutely zero false positives. I have seen a handful of spam messages come through, but it's in the single digits over the last two years.
That's often the case with one of my friends who has a Gmail email address: anything that I send him (once or twice a year) doesn't even show up in his spam list without first making him start an email exchange to me => in my opinion that kind of filtering is just too easy to do (come on Google, at least put it into the spam folder and/or show it as a colored/blinking line and/or put some warnings whatever - don't just delete it), and of course it poses questions about oligopoly etc. How Gmail works is just not fair (in my opinion) :(((
Spam filtering on Fastmail has been the same/better than Gmail was - including possibly fewer false-positives on the Fastmail side. Gmail was getting worse about those, both with mailing lists and individuals' emails.
Like many on HN, I'm diversifying my data/access risk across more providers. Too many wake-up calls recently about Google locking people out. There are still some services Google is compellingly better enough that I still use them (Android, Maps, YouTube, Google Sheets), but Email was too precious to tie up with them. I also wanted to finally kick myself into stoping using the `gmail.com` address at all anymore (maybe 20% of my emails before the migration).
One important point here, and I don't know how long you used Fastmail when you tried it, is that Fastmail uses a personalized spam filter. It probably took me six to eight months to receive enough spam on Fastmail to actually train it. (In the interim, they use a non-personalized filter, which as I said, still worked!) Gmail doesn't seem to be able to make personal spam decisions: When I was regularly using Gmail, some types of regular messages would spam-bin no matter how many times I marked them not spam or classified them as a particular category of mail.
Anybody that knows better, will avoid Gmail like the plague or use it for only the minimum.
Gmail is worst-in-class at this point. It's slow, it's bloated, it's bad at spam filtering compared to the alternatives. They've been riding on people remembering email before Gmail, but not really actually stayed competitive with any modern alternative offering.