main problem in voxel engine would be animating objects as phibermon pointed. here's pretty good explanation why http://www.youtube.com/watch?v=gNZtx3ijjpo (maybe in other part, my connection is slow so couldn't check)
btw anyone played ID Software's Rage? terrain there was rendered using voxels if I recall correctly. Maybe future is mixing both polygons and voxels as they shine in different domains.
Still for our 'indie' purposes high detail voxel engine is no better than traditional one. both take a mountain of money to shape decent content