SHOCK! :O
Are you playing RTR on easy mode?! I always play it on hard or very hard mode, and the AI is FAR from stupid!
My battles in RTR are some of the most bloodiest ever, and especially Greece, Macedonia, Ptolemaia, and Seleucia are always killing the shit out of my Legionaries and the Ptolemaic Empire must LOVE Siwa, as they're always charging with stacked after stacked armies at it almost every turn!
Of course, this is after I edited the army upkeep to make it much lower.
Sarmatians suck, that's why their formation suck. They're friggin barbarians, not even disciplined enough to gather in packs like Gauls and Germans to wait until they're together to charge at the enemy.
I don't know if Barbarian Invasion is realistic at all, but I don't like it at all. Apart from my instinctive hatred of the Medieval era (and the close proximity of Barbarian Invasion era to the Medieval times), there's simply too many barbarian factions that all look the same to me, with a few random faction-specific units, but otherwise the same. The Huns suck, as they're all cavalry, and not even that great cavalry either. Both Roman Empires are too big, the leaders too corrupt, and the Western army is too barbarian, and the Eastern empire is too undeveloped almost everywhere but Constantinople and Athens and Thessalonica
I agree with you on the Sassanids. I read how great they are in the forums, and how everyone's talking their gamer trash "cilbs pwnx0r!!!11!1111" and when I play it they're as boring as Parthia. And they're hard to deal with, too.