In our glider planes we had an IAS marking for VNE, however in the POH was a table of pressure altitude vs VNE which showed a marked decrease of VNE with rising altitude. Flying the marked VNE or close to it at higher altitude can very easily lead to flutter and partial or total destruction of the airplane. Probably more of a problem with gliders than with most SEPs as gliders have no natural altitude limit apart from pressure breathing limitations, flying into the 300s to 400s is very much possible given the right weather conditions.