The worst rotorcraft driveshaft failure example I can imagine would be the CH-47's interconnect drive between the fwd/aft rotors.
The 1982 Mannheim crash is a very graphic example of what happens when the interconnect drive system fails on a CH-47 tandem helo.
The most complicated rotorcraft driveshaft system I can think of is the V-22 cross-wing interconnect. Three gearboxes (2 TAGB & 1 MWGB), a dozen or so driveshaft sections, a similar number of hangar bearings, and a large number of diaphragm type flex couplings. What makes things difficult with this particular application is the driveshaft system must accommodate flexing of the wing structure. The V-22 interconnect drive system's primary function is to power both rotors during OEI operation. And failure of the drive system during certain OEI operating conditions (hover, transition, etc) can be catastrophic.
The 1992 Quantico crash was an example of a driveshaft failure during transition and OEI conditions.