I think the risk is pretty low because Fastmail has a separate logins for mail etc so you can disable that if you are concerned.
It says everything happens on device and it’s privacy focused.
The lack of a business model is concerning but I know the desktop third party email clients (FMAIL) are also free. Maybe it’s something the developer wanted etc. the motivations are unclear.
I am not recommending this app because I have not tried it. I am merely sharing it hoping others can share their experience.
Nothing, if you have only a Fastmail account. Otherwise, you must either forward other accounts to Fastmail or use another app for your non-Fastmail account(s).
I used the former approach recently when trying Fmail on the Mac, but decided against that for organizational purposes and am now back to stock Mail on iOS and MailMate on macOS.