Yeah combining FMCB (or FHDB) and modchips is a hit or miss.
I assume if you have a Matrix chip (or a clone) you wouldn't need FMCB to begin with but in my case the chip is a really old one that only allows playing burned discs so I still need to use softmods for OPL.
I wouldn't be so sure saying wouldn't need FMCB. DEV1 needs pressing and keeping pressed at boot joypad. FMCB autoboot to the same menu but with possible 99 applications on a list. Quite a game changer for me.