RTW AI same as MTW AI

[Deus ret.]

The R:TW AI goes after the primary attacker/defender and ignores reinforcements until they get into view. It is sometime since I played M:TW, but I remember a few battles were the AI would always go for the weaker enemy army and destroy it before the stronger was in a position to help.

AFAIK MTW didn't know several armies attacking from different directions, that was introduced in RTW by dividing tha camp map into tiles. So usually it didn't have to cope with more than one group of enemies.

I've never really been a fan of custom battles though, it might be different there.

It never just backs up onto high ground in the rear during a sally.

Yes it does. At least if you have onagers/long range attackers inside the walls, and sometimes also if you haven't.
However in general the RTW AI can easily be outsmarted especially during sally battles.

Instead of being unhappy with the lame RTW AI, you could as well ease the annoyance. There have been efforts to alleviate this state: Try e.g. the Darth mod. I even had fun while losing battles thanks to the nasty AI.