I have played a few games with zerg and have had some difficulty running the 'ground pounder' route.
When I say ground pounder, I mean defilers and ultralisks. They have a lot of difficulty when in facing numerous flyers.
I suppose that I could invest in zome scourge to counter this, but I find that I am strapped for gas with defilers and their technologies... plus using hydralisks to counter air. Hydralisk cards are few and far between.
It seems like I am forced to go the route of air with the zerg to be really effective.. there is no real viable A.A ground units where both the Protoss and the Terrans have several options (protoss with archons and dragoon upgrades, terrans with goliaths and the fact that marines can hit air which is just super powerful).