Well two things, but title word limit.
1) They could be brought up to speed by actually incorporating them into some technologies other than the 1-2 techs they only appear in atm
2) But the greater issue atm is that they're just a really cool unit who could have been made much more unique, but CA just made them a generic AP missile chariot that focuses more on missiles than melee. It's a shame they can't deploy and turn themselves into barricades to protect your infantry. Or the repeated rifles they wield actually fired like repeater rifles.
But never ever say War Wagons are weak. They were weak on release. But they received substantial buffs and are very powerful now. They trade Handgunner's raw DPS for mobility and tankiness, they maintain their max DPS even after taking incredible punishment yet keep all 3 models, where Handgunner's and Outriders would have long lost several models permanently taking that same amount of damage. They're just.... incredibly boring