Right now, dismemberment is based on damage, and with mod_skillmode on, you do less damage to certain areas, which might lower your chances of dismemebering someone.
I'll see what i can do to boost the odds.
Dualsabers should have no effect.
The f_dismembermod cvar is not in use yet, it will be hopefully be functional in a future version. Make sure it is set to 0 in the meantime, just in case.