In campaign mode id say it depends how you build your army to counter them.I play as selecuids. half sythed chariots and half cataphract combo has yet to lose me a single battle.I think I've taken on armies 3 times my size and crushed them. Romans should be rightly afraid of 3 things. Elephants,Berserkers and chariots. Romans have to semi prepare on how they plan to combat these threats.Phalanx armies have no trouble with these problems.
Also in campaign mode Phalanx armies Have GODLY defense of city streets and squares. In my selecuid campaign I literally routed thracian and macedonian armys at least 10 times in the one city. I think its where constantinople should be.

In multiplayer me and an ally defeated some romans with all phalanx armies. He took spartans and i took sacred band inf.