Empire has the best setting and scope of all the TW games to my mind, but there's probably a more pressing need for an Empire II than a Rome II (Napoleon doesn't count - sure it made the game engine work, but it had far fewer factions and a much smaller theatre of war. Napoleon was more akin to a Fall of the Samurai-type standalone expansion than a separate entry in the series)
Nevertheless, I've never encountered too many of the bugs, and the AI's bad but not noticeably worse than Medieval II's execrable AI - for every siegeless Empire army I've had wandering aimlessly around my fortifications when they should be scaling the walls, I've seen Medieval II AI hold everything back except the catapults that advance to get killed, one at a time no less (and then -only when they're dead - doing the same with the battering ram), or a Fall of the Samurai army attacking a superior force in a city when they have a calculated chance of winning well below 25%. Yes, all of these are real examples. The India campaign in particular needed to be better thought-through though - I like the Maratha as an addition, and they have some nicely varied units, but there are no player factions to compete with them, and naval warfare, the multiple theatres of war (the game's two key innovations), diplomacy and to a large extent technology are essentially irrelevant; at the very least Persia should be either a playable faction and/or a much stronger, more aggressive AI (in reality it was the Persians who defeated Maratha territorial expansion).
Adding a tech tree for, well, tech rather than just buildings/units was a good addition to the series - the trees themselves could have done with pruning, but not to the degree they received in Shogun 2. Gameplay dominated by ranged units and cavalry does make battles play very differently, and also allows the use of more varied formation tactics (sadly many of these come somewhat late in the tech tree - authentic for such things as infantry squares, not so for wedge formations) than just 'guard block' or 'skirmish line'