Mostly the noise as i don`t believe a well tuned diesel car doesn`t need/has to be noisy and belch smoke to be powerful quiet the opposite if it is well tuned. SMF will also tend to vibrate more and doesn`t do the gearbox mechanism any good plus the transfer box in my case. I went SMF on my car when my last clutch died which was good for 400lb torque and DMF was still in good condition with no free play when removed and had no choice but to go SMF if i wanted a stronger clutch as no company i know of makes a 600lb clutch that fits a DMF.