
So I was a high level diamond Zerg before masters league, and than took a break and haven't managed my way into masters yet, I have around 450 games as Z under my belt (excluding beta) and they have always been my race since BW.
However upon my return back to the ladder, I started smashing everyone, untill I hit a huge wall, I mean I went from 30-4 to 1-12, it was ugly, the funny thing was almost every game I lost I was extremely frusturated, and felt I was the better player, now I mostly chalked this up to the lack of recent time on the game. But loss after loss came and I felt I was much better of a player, just one micro mistake with my banes, and bang, my chances of winning went right out the window. I would rage at the opponent calling him a F******* F***** piece of SH*T, and not enjoying the game even some of the wins were unsatisfactory
I decided to try out protoss for a night, I started by practicing BO's vs computers, then moved to custom games and ultimately to ladder, and even though I still lost quite a few as Toss, and won a few, after every game I was certain it was my own fault I lost, I didn't scout, or didn't micro/macro well, made the wrong choices in general. So after every game I lost I politely GG'd, maybe ask a few questions about the game how to improve and than leave. I felt very rewarded when I won, and never felt unfairly punished when I lost.
Now the reason many Zerg's like me feel this way is not always due to actually imbalances with the game, but the other races are easier to utilize i.e. no creep spread, injects, etc.... also they are more forgiving, and in general more fun due to greater options and control of the game. These DO NOT make the game imbalanced, but rather a more frusturating experience for Z player.
For example say a Terran 2 rax a Zerg, the zerg now has to scout it, properly defend, and stay macroing well to win, this is alot of stress and when you see the Terran opponent put up bunkers and auto repair, it makes his micro task feel much much easier. Another example say your late game ZvT your unit comp is ling/bane/muta vs the standard Terran marine/tank/thor, now there are 2 possible settings here, one burrowed banelings are blown up by one clutch scan, GG. Second you have your army ready for the surround but you rely on the banes to seriously deplete the marine numbers so your mutas can magic box the thors, this is totally dependant on how well the Terran micros the marines to judge who will win the fight and ultimately the game. There are obvious things a zerg could do in these situations to come out ahead, but how I wrote them is HOW it feels whilst in the middle of the game.
So it is not really the balance that stems the frustration of the Zerg race but rather the seemingly lack of control of the game and options and the stress of such a demanding race
PLEASE don't turn this into a balance thread, that is not what this is about
thanks

tl;dr
Zerg isn't underpowered just the other races are easier to utilize i.e. no creep spread, injects, etc.... also they are more forgiving, and in general more fun due to greater options and control of the game. Leaving greater stress on Zerg players than the other races
cheers
scott