Apart from the AI not being able to deal with it, there is another thing bad about the campaign map and that is the number of possible battlefield generated. Each and every land tile that is not a mountain, dense forest, river or swamp has to be a battlefield: there are thousands of possible battlefield and since they are all generated by the same algorithm they are all quite dull.

S:TW and M:TW on the other hand had "hand-made" battlefields with small hills and little forests you could use to hide troops in. Especially the S:TW battlemaps were very good in terms of placement of these features. On the other hand, R:TW battlemaps tend to be dominated by one big hill or one big forest and that's it. Tactically that is very dull, since there is only one feature you can use (and then the AI doesn't use it well either).

Though the 3D map is very beautiful, the 2D map wins for me if it means the battlefields will be more diverse.