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.
I used to be a fan of the Total War games, back from the first Shogun Total War in 2000. The AI was never that great but was good enough to make the game fun to play. The main thing was the battlefield mechanics worked well, rewarding smarter tactics, like flanking manoeuvres, which could allow a smaller force to defeat a much larger one.
The original Rome Total War was probably the most fun and Medieval Total War was enjoyable.
After that it just went downhill, adding pointless features while worsening the battlefield gameplay. The first Warhammer Total War was where I tapped out - the limited size of battlefields meant it basically became a tower defence game, fought in three columns.
Reading this, seems like they became a victim of their own success.
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.