Are Z-shaped cranks a good idea?
If the aluminum is sufficiently stiff it makes zero difference -- the crank could be any shape (a disk, an S shape, etc), but the relationship between the two contact points would still remain the same, and that's all that counts.

The only effect the crank could have is adding a bit of spring to the crank, which might be good or bad for effective cranking.  But aluminum makes lousy springs, and if it flexes much it will soon fatigue and fail.

The risk to your body parts is getting cut by sharp edges when the crank fails.