Tau Zombie squads basically have the firepower of a basic foot laser platoon. They gain the ability to be crammed into a single ton of cargo space, but do so by sacrificing almost all ability to take damage. They've got armor and damage reduction galore, but all that stuff still won't reduce a hit below 1 damage, and they can't take more than a couple such hits before the squad is crippled.
Oh, and they've got the range and accuracy of a large laser tied to a targeting computer.
So they're really easy to haul around, have VERY impressive range and accuracy, but die really fast if anyone pays real attention to them. Honestly, I'd put regular troops or BA in our proverbial Zhukov, and keep the Zombies further back where they can act like numerous tiny field gun platoons.