If you have physical access and a flash programmer they're not secure at all. If you're trying to stop a random passerby from messing with your firmware settings they're great.

The BIOS does control the boot order sequence for instance.

I guess if you have access to it, you could force boot from a malicious USB stick, or the network, that would simulate the disk decryption prompt.

I guess you could also remove security measures that your company put in place to e.g. prevent the usage of USB to prevention information leak.

One is supposed to prevent access to the hardware, one is supposed to prevent access to the data. They're not meant to solve the same problem. But that say, you can only block access to hardware so well, so it's worse in that regard.

well, you could make the machine boot on another device and potentially trick someone into entering their credentials on a "phishing" screen.

They are not secure at all. Full disk encryption is the only way to protect your data in case a stranger has physical access to your device.