its not just that, USF get zooks in the same upgrade as BAR's and with the Captian cheaper we see that every game too + quicker AT guns.
Its all-round less effective. And nowhere near worth 55 fuel.
Flack halftrack is not an offensive weapon. It's a deffensive / attack support one. It should never be sent forward and be the spearhead but supporting advancing infantry from afar, behind them.
More than this, as simple as OKW faction seems, the OKW onset must be adapted to the situation on the battlefield. I don't have a default OKW start. Sometimes is a good choice to start with the Puma building ( I don't talk about tiers, t2 or t3 means different things for different people) sometimes a better choice is to start with the jagdpanzer building. If your soviet opponent chose the tank hunters doctrine for instance, starting with Puma building may prove to be the wrong choice.
To conclude, I don't think flack ht is UP. I think that is a high risk / high reward unit. You lost it in the startgame, you have a big chance to lose the game. You keep it alive, the meatgrinder that it has, will shred and supress the enemy infantry causing enough damage to boost your winning chances skyhigh.