I had an issue last year with it not detecting or learning a particular Russian spammer but as of ~4 months ago they all now correctly go to spam. Otherwise it's been very good.
Their AI or whatever algorithm they've built for spam (or in this case, phishing) is getting worse and they have no incentive to care. Gmail users aren't leaving Gmail over this and their market size means everyone must cater to Gmail.
Somewhat related but I finally relented this week and now route my self-hosted email through SMTP2Go thanks to a tip from another commenter here. Over the past few months I've noticed an acceleration of my email going to Gmail user's spam folders and just couldn't deal with it anymore.
Perhaps given how big Firefox is they actually prune this one.
These types of companies are really interesting to me, I had no idea they've raised close to $1B over 5 rounds/7 years though[1]! Curious what they've needed that for, I would presume they've sold quite a few of these plus the whole subscription model.