I have the exact opposite experience and find Fastnail's spam detection quite superior. There is the occasional spam.email getting through - maybe 1 per day - despite the fact that I have quite a number of custom domains with catch-all email addresses.

At the same time I really never have to check my spam folder for false positive, everything in there is literal spam so I forget checking it for weeks on end. Whereas on n Gmail I always need to check the spam folder, Google completely overdoes the filtering it and a lot of wanted emails end up in spam.