Flatpaks Safe?

Hi, I started dual booting between linux mint and windows 10. I'm currently using Mint 20.3 Cinnamon. Im still pretty new to linux so bare with me.

My concern is how safe is it to use flatpaks? I plan on having sensitive data on my PC like my password manager. The flatpak that I currently have installed is discord right now. From my understanding the discord in flathub isnt being mantained by discord themselves. So in the case if someone malicious tempered with the application would that affect only discord or would it affect the entire OS? Also is there like a rule of thumb on which packages to go for first? Appimages, Snap, Flatpak, or Native?