If you want an Iberian unit to go toe to toe with legionaries (or at least the pre-marius ones), that's what the Scutarii/Spanish Mercernaries are there for. Unfortunately Carthage can't train them by default, although there is a texture already made for them so it's not too difficult to add them (although you have to make your own unit card pics).

I don't mind the actual Iberian infantry unit being weak, although I've lowered their upkeep to compensate for their lack of ability.