The problem with a consistent challenge rating is that if you run into opponents that you find too hard (even with close to the best available equipment), you can not go level up somewhere else and return, because they will then be stronger, as well. Early in the game you might be able to level and adjust your stats/skills to suit problem opponents, but that would be much harder late in the game.
If the game detects you are having trouble and make opponents weaker for you, then I don't see any point in allowing character builds, if they are all going to be equal.
I can see partial level scaling to keep weak opponents from getting to just be a nuisance, but IMNSHO strong opponents should not be nerfed.
Theoretically, dynamic difficulty scaling could work well in a shooter or action game where the focus is on the skill of the player, but once you start building a character with a choice of stats and skills, those choices should matter.
