If you wrote a BIOS version of 1.0.4627.X or lower to a 1.1-1.4 motherboard or a BIOS version of 1.0.4817.X or above to a 1.0 motherboard then the result would be a "coma console" because 1.0 and 1.1+ motherboards have a differing MCPX ROM and use a different encryption algorithm for decrypting 2BL into memory (1.1-1.4 boards also have an extra step in the bootchain)
All official retail Microsoft BIOS' are 256KB so if you wrote a 1MB BIOS image and it wrote at least 256KB of the image to flash then that's technically okay but if you wrote a 1.0 image to a 1.1-1.4 revision or vise-versa then 2BL can't be decrypted properly and that's why your console is 'bricked'. You can recover the console by installing a modchip and I believe you can recover the TSOP as well but I'm not the one to answer how to go about that
Quick rundown of critical failure states on the Xbox:
Coma console: An error in 1BL or fatal hardware failure (CPU, NB (imbedded in GPU), SB (MCPX), RAM, SMC, or TSOP, and anything in between connecting those chips.. so basically anything) - no code to be able to signify an error to a user
LED blinking: An error in 2BL (or FBL/2BL on 1.1+ revisions) or fatal hardware failure (RAM, GPU, or EEPROM) or something wrong with the kernel image - can blink LED's with code to write to SMC
Error screen: An error in kernel or something wrong with higher level buses, devices, software, or basically any chip on the board not behaving correctly when the console is in a fully initialized state
You're correct in saying BIOS' and kernels are usually separate things, the Xbox doesn't actually have a "BIOS" but that's what the modding scene labeled it as a long time ago - it's more so firmware. The Xbox "BIOS" is 2-3 separate blobs of code mushed together into a single chunk of data stuffed onto a flash chip (or ROM on 1.6)
BIOS = 2BL + kernel
or
BIOS = FBL + 2BL + kernel (in the case of 1.1+ revisions)