My experience was that Fastmail has regular hiccups, they just don't make the news. I left for Google Apps years ago mostly for that reason.
I've used them as my mail provider for three years or so without noticing any. So if they have regular hiccups, most of them can't be that serious.
Been a customer for over a decade. Can count the number of outages I've actually noticed on one hand, and they've all been very minor.
I'm surprised that anyone who thought Fastmail was a good choice would even consider Google as an alternative.
I chose Fastmail for business, as an upgrade over self-hosting, because it seemed to be a solid provider, not because "Google, ew!" Turns out, Google's the more solid provider (three years as an FM customer before reaching that conclusion).