Stability in the wind varies a lot depending on how fast you ride and the shape of the wide rim. I have had three sets of 60mm wheels. Two are fine in the wind and the third is a handful at the transition. The difference from most 30-40mm rims to 60mm rims is only 3-7 watts at 30 km/h. There is no harshness difference from 60 to 30mm carbon or to aluminum for that matter.....tire and tire pressures is what controls that.
Carbon rims often have a very wide internal width compared to aluminum rims (say 25mm vs 15-17mm), this "plumps" up the contact patch of the tire and the ride is "smoother".....that might be the increased comfort of carbon of which you are told.