Spotless. I've gotten maybe one false negative (i.e., the spam hit my inbox) and no false positives.

I have a very different experience. I've had my fastmail account for ~5 years and get 20 new pieces of spam in my inbox daily, while on my gmail account, that I've used for ~10 years, I only get 1 spam per month on average.

Fastmail's spam filtering is atrocious in my experience.

Works fine for me. They also have flexibility in how spam is handled, and you could even forward to gmail and back if you really wanted to use its spam filtering. https://www.fastmail.fm/help/spam_virus_protection.html

A few months ago I had to drop my spam threshold from 5 to something like 2.5 -- my spam issues cleared up immediately.