Boot any Linux ISO and bypass any OS logins (secureboot would prevent this), connect devices over IO to otherwise exploit the login or just dump RAM content to potentially get data or even keys.
I would never trust OS login over FDE especially on desktop.
And then what? They will be faced with a login prompt. Reading from disks is useless. My login password is strong.
Boot any Linux ISO and bypass any OS logins (secureboot would prevent this), connect devices over IO to otherwise exploit the login or just dump RAM content to potentially get data or even keys.
I would never trust OS login over FDE especially on desktop.
EDIT: first point won't work on second thought