The AI was never good, but before Rome II it seemed like it either got a little better with each iteration, or at least, the parts that got worse had to do with incorporating new mechanics (Empire's AI was terrible, but it was pretty obviously because trying to jump from armies with swords and arrows to those with muskets and cannon was a big change).

Outside of the navel stuff, Rome II's combat wasn't wildly different from Rome I, Shogun, Medieval, etc. This made it kinda jarring what a step backwards it ended up being.