In RTW vanilla, if you keep a solid line of phalanx, you can walk over hastati with virtually no loss. Keep the enemy off your flanks and the phalanx wins easily. I can't imagine that EB has changed this, as it seems historical and otherwise the phalanx is pretty useless.

AI phalanxes need all the help they can get, as the computer keeps turning their flanks to you, inviting their piecemeal destruction. In some ways, I think EB should be balanced not to be "realistic" per se, but so that the game gives realistic outcomes when a human fights the AI. For example, there might be a case for inflating the combat values of phalanx units to make them more robust.

More generally, I sometimes wonder if the "realistic" combat system in RTR 6.1, for example, may actually make the game worse. In this model, cavalry and missiles only really work when attacking the flank, and heavy infantry needs to be flanked to avoid fighting long and hard. This is very realistic and I have a hunch EBs combat system may have similar features. But since the AI can't really flank, it's giving the human a big edge over the AI and so leading to potentially imbalanced and unrealistic outcomes.

It reminds me of those great Talonsoft wargames, East Front and West Front. Reluctantly the designers gave the option of assigning lower armour values to tanks' rear and sides. Of course, that is realistic. But they told you not to use the option against the AI, as it did not protect its flanks - allowing you to unrealistically slaughter "monster" tanks.