as a zerg player, I found the single player campaign pretty easy. Granted I only played it on Hard. But in WoL on Hard I at least failed some missions. in Hots I didn't fail any missions and ( like many have said ) face rolled everything with roach/hydras. The f2 button makes the SP experience way too easy imo.
All I ended up doing was making 2 extra hatches and maxing drones. Then just making corpser roach and hydra balls to win. When you get kerrigan's drop pod the game becomes ridiculously easier.