I quite like the simple design of this app, but like @nowakowski mentioned, the application is seriously having some issue, with version 3.34.2 running on linux mint 19 cinnamon. So far I only got it run once, that's when it got installed. After that it just gave me a blank screen and the application just got frozen. [Update] I found a workaround. I went into the "System Monitor" and kill Geary process and re-launch the app and it works. This has to be done everytime I restart the computer. Found another issue after this, I notice that Geary does not support Chinese input.
I really like the application except this bug the reviews before me mentioned. I found one thing useful, until this bug is fixed: If you run the application from the console first with flatpak, it works perfectly fine.
Something is wrong with this version. Sometimes a list of emails appears, sometimes the application freezes on an empty interface.