https://security.stackexchange.com/questions/267222/full-dis...
So, there are two scenarios here.
First, PC with FDE + normal boot gets stolen. The attacker cannot get the data without the password, so it's safe.
Second, unattended FDE + normal boot PC gets tampered with. Attacker manipulates the bootloader. Unsuspecting user later boots the tampered PC, unlocks the FDE, gets owned.