Good luck. I did the same and now I have all sorts of issues when employers send Calendar invites to my (ex-Google, now Fastmail) email address. I cannot accept them due to weird errors, etc. Not the best look if you're ostensibly an Operations guy. :)
We're not aware of any general issues like that. Please open a support ticket with the full details and we'll happily investigate: https://www.fastmail.com/action/support/
I don't use Google services for anything work related (we're government so we have "good old" Outlook and Exchange), just personal stuff and side work. Basically, as long as it can sync with the calendar and contacts on my phone, I'm set.
Thanks for the warning though, and maybe one of the FastMail devs will see your post and address it.
