Hand flying smoothly with precision for more than a few minutes in the higher flight levels is all but impossible
It should certainly not be impossible to a competent instrument rated pilot. After all, it wasn't all that long ago RAAF pilots flying fighters in the old days regularly flew in close formation in cloud at high altitudes with no automatics. The Canberra bomber was a single pilot aircraft that had no autopilot and regularly flew at 45,000 feet for four hours often in IMC and without the benefit of weather radar. Todays military pilots are taught to use all the whizz-bang automatics available yet their training still strongly accents manual flying skills which may be needed if the automatics are knocked out of action. I guess it all depends on your training and personal flying ability.