The original bushings were somewhat hollow, but that's part of the design. The hollow areas allow for minimal movement side to side, and a little more fore & aft which softens the shock when power is applied. Most subframe bushing failures are fatigue related, where the center metal alloy ferrule separates from the rubber, which allows far too much movement, and misalignment of the subframe.
I don't think the voids make them failure prone; it seems like we're seeing more failures now, but the vehicles we are dealing with generally have upwards of 100,000 miles on them, so failure at that age shouldn't be a surprise. What bcredliner said is true; few of us will replace the bushings twice.
The poly bushings differ from stock in that they have no voids; I don't think that's really a positive or negative attribute, just different. It'd be interesting to see what a set of poly bushings looks like after 150,000 miles though.