There were some hugely unbalanced features in the initial release. For example, horsemen got 100% retreat chances. Everything was fixed by the final patch of Vanilla. The game wasn't bad but there were some errors in the implementation.
In contrast, the whole Civ 5 game concept simply sucks. They've improved it a bit but it's still boring, largely because the "improvements" are all about nerfing exploits instead of adding fun and challenge.
Check out the contrast between the Civ 3, 4, and 5 war academies. In Civ 3 and 4, the articles are mostly about how to grow your empire. In 5, they're about the - incredibly unbalanced - characteristics of the various civs.
Still, after this expansion (maybe another one afterward) it could be playable heavily modded.