I was looking at a crankshaft on Friday evening that was split into 2 pieces. The owner had switched to a SMF from a DMF as they were concerned the DMF couldn't cope with the horsepower increased (Supercharged R36 Audi TT).
They firstly got a strange resonance from the SMF which they hadn't suffered on the DMF, and this alerted them that there was an issue. However the noise got worse, and it became apparent that something was really not right.
So I have been led to believe (opinion, NOT necessarily fact), part of the benefit of using a DMF is that it allows for an element of imperfection in the system (as parts like the crankshaft are no longer made to the quality we would like to think they are due to high volume production and value engineering). The DMF protects us from this and offsets any vibrations in the system. However once you remove this you remove an element of safety net, and the crankshaft is exposed to greater stresses as any imperfections aren't ironed out by the DMF. If you ramp the power up considerably 450bhp+, the effects of this get worse and worse, and ultimately can cause the crank to snap if the resonances/vibrations exploit any weak points. Whilst it won't happen to everyone, it's one of the risks of running a SMF. The DMF is not really built to cope with enormous stresses from highly modified cars, so in some cases the SMF is the only route to consider, but in the case of the TT above, the power was moderated back, and the SMF replaced with a conventional DMF.
If you have a SMF and you are reading this, please be aware, a failure can happen on any car with any level of spec, so this isn't an attempt to scaremonger you or say you've made the wrong choice. It's just a 'view' on the world based on some fairly decent research. The SMF being run at the time was a Sachs kit, and they now have this crack back to investigate whether its a purely isolated case (so I am led to believe).
If I hear any more I'll let you know. I'm still running OEM clutch and DMF a I don't intend to break 350bhp.