Yes, seen them all, and in just about every case it's the same problem not understood by the user ... and it's not just on ASUS devices
The function keys serve 2 purposes - as F1-F12 keys and also as a multimedia control (sound, screen, etc). In cases where the function key fails to activate the process, it's because the user has left it in multimedia mode i.e. on my ASUS, F2 can either function as an F2 key or turns the volume down. If left in the volume mode, F2 boot to BIOS just ain't gonna work. No use blaming the computer
(Fn+Esc toggles Fn keys on/off)
As a rule, when wanting to access the function options, I always have a USB keyboard handy at boot up time, forget about the device function keys, you wouldn't know what mode they were last in. The multimedia control option can be disabled altogether in the BIOS