I fly three or four Cat3 a year in Europe, when we are flying at all, that is. I flew circling NDBs in Ansett when I was based in Perth.
No question NDBs with circling are both harder and require more skill and more moral fibre in real life.
Cat3 s are harder in the sim, because you get asked about so many rules, and there are so many variations on go-arounds etc.
So, anyone who says Cat3 is harder, has more sim time than line experience, or has spent their life ILS to ILS, and simply has t flown a circling approach.