It's all relative.
My 165 cm SL skis tend to want to wander a bit, kind of like a wobble, when flat. That is likely the "instability" folks are talking about. If you have a long radius ski to compare to, you will easily tell the difference. However, my 13m radius skis are stable in the sense that they do not vibrate uncontrollably like SL skis of old, even at speeds in the neighbourhood of 60 mph. Vibration-wise they are quite stable. The only danger is that should the skis suddenly (and momentarily) hook up when making a SG or DH turn at a mile a minute something has got to give. Very dangerous. Get some speed skis if you want to ski at 60 mph. I now tend to keep my SL skis under 40 mph where they can carve clean turns.