While authenticator apps can still provide a secure layer of authentication, YubiKey’s hardware-based design and unique features make it a more secure option for multi-factor authentication. Since Trading 212 is a investment app - I think it would be awesome to have this advanced security feature (for more advanced users).
The problem is - when you get your phone stolen the thief can access the app without much issue. Also, would be awesome if app demanded you to use Trading 212 password when Face ID/Touch ID fails after 3 attempts. (something like Proton Pass)