I'm using FMCB since day1 and had a lot of testing to do, especially when the disc-detection-routine was created and the system-init was incommplete.
Those two things are the only things, where I can imagine a fault caused by FMCB.
Still,... I never saw or heard of any issues like that, caused by FMCB.
PS2 V7/DMS3 V2 (FW:2.4b7); Seagate Baracuda 200GB
PS2 V7/CC1.0 (FW:34 hacked v2 BM:2.1.6); Maxtor DiamondMAX9 PLUS 160GB
PS2 SCPH-30004R; NoMod+NoLaser
3xSony MC 8MB
MAX/Datel 16MB with Boot-CD
Custom FMCB 1.8b+ Beta-Build, my AIO 0.5, Sony&xRhino-Linux