Welcome to the org Passion, enjoy your stay

Indeed its as you say especially when they have to break formation and that is easy to do against the AI. If you have fast troops like barbarian infantry (bastarnae, thracians, falxmen, naked fanatics and the like) or cavalry combined with some tough heavy infantry, legions or heavy swords or phalanxes of your own, then you can spread out the phalanx instead of fighting concentrated in order to take advantage of wide range of attacks at flanks and rears. It takes more micromanaging though and you are running teh risk taht your troops may not support each other in terms of numbers locally and so be of low morale.

Triarii can nearly beat the phalanx or thereabouts, but principes can also stand in guard mode and deep formation against halanx for long time allowing you to flank. If only hastati are avialable they will have to do, just you have a bit less time for the flank attack before the pinning units break.