Been reading that races are not balanced very well, specifically the Zerg. Seems a lot of top Zerg players are sick of it and are changing? Kind of sucks since I really like the idea of Zerg, but Protoss are my next fav. Always seems human based races in Blizzard Strategy titles are favoured and perform the best.
I have yet to play online, will do so once I am done with singleplayer and some practice vs AI, just to get a feel for things.
I wouldn't say Zerg's units are unbalanced, but its skill difficulty is much higher and very unforgiving compared to the other races.
If you compare queen's spawn larvae ability vs chrono boost and mule; if you miss an injection, the Zerg's production gets hit hard, whereas with the other 2 abilities you can chrono boost 2 different buildings or summon more mules. Spawning larvae don't stack.
Zerg are terrible off of one base hatch and they need 2-3 hatches to stand a chance. Therefor they need to macro and expand like crazy and this adds more pressure on the player to manage and defend more than one base. This is more apparent in 2v2, 3v3, 4v4 as I'm usually forced to build an in-base hatch to keep up with my allies.
Zerg units also require a lot of micro and good positioning to be effective. You really need to put effort in to get that surround or you will lose your entire army. You also need to rely a lot on getting the opponents forces out of position, back off if they're in a choke and wait for them to make the move.
Zerg can build a large force very quickly, but when fighting against a 200/200 terran or protoss, they will lose. So they have to remain on top of their macro all the time to be on even ground. If zerg lose one unit, they need to immediately replace it. They cannot queue up units like the other races and have to select hatch, select larvae and then select the unit which means more APM. (easy with hotkeys, but I find Protoss warp gates much easier)
Then the last thing is that Zerg will never set the pace in a game. They are a very reactive race because they are able to tech switch real quick and create an army very fast. Ideally as Zerg, you want to drone up until he pushes out and only then create the army to hard counter the opponent. Though, this requires a lot of scouting, good knowledge of the other races' units and builds, all so you could counter what the opponent is doing.
The learning curve is a lot steeper than the other races, but it feels so damn good when you pull it off. Though, if the Terran or Protoss player plays a perfect game and makes no mistake, Zerg will lose all the time. Its all to do with the mechanics of Zerg that makes the other races harder to crack.