The challenges in Soul Calibur 2-4 were much more interesting (and varied). But the game mechanics of SC allow for more leeway when coming up with different gameplay (wall-bounce combos, unique fighting styles per character, ring-outs, etc.). I think NRS might have come up with some more interesting ones, but we can hope for more with DLC.
Really, it's just amazing what they were able to come up with even after designing a whole new fighting engine plus all the other kontent they loaded the game with. I'm not about to look a gift horse in the mouth.