The sky crane has all the problems of a conventional lander, plus the issues of a compound pendulum, plus the issues of the payload interacting with the rocket plume, plus the issues of the Martian wind interacting with the whole assembly, .... need I go on?

..would have thought that hare-brained schemes (with hair's breadth error margins) had gone out of fashion after MPL (and beagle) and the discrediting of FBC. Optimistically, i've hoped that with MER, we're *permanently* back to sensible designs, carefully validated ..
Shouldn't these guys know the difference between what's *possible* and what's DO-ABLE? That must be basics101 for any engineer, let alone a planetary mission designer