These discussions always gloss over the reliability of delivery (i.e. is your email making it to the intended inbox or getting squashed into spam, or even shot down before making it to the recipient), especially when custom domains are used (which I suspect is often the case with Fastmail). As much as I want openness, independence, portability, etc of my personal digital stack, sometimes "just getting the work done" takes precedence, and email is one of those areas if not THE area where that matters most online.