Kingdoms of Amalur was very fun for me but extremely easy (even on its hardest setting). Witcher 2 is definitely more challenging on dark mode than KOA but doable if build your character correctly and use potions/oils correctly. There is no playing after you beat the game so if you don't get level 35 before the end you have to start a new game or load a save a ways back and farm experience. It's not hard at all to get level 35 though if you try.
Overall this game isn't as open a world as Skyrim but neither is KOA. They both have connected regions that you can travel to. In the end, the Witcher 2 is more of an adult game while KOA is for all ages. Both are fun to play though.