Siege Tanks in critical numbers can one shot large clumps of units, and they require no micro.
I disagree with most of your post, but this is by far its weakest point in my opinion. Siege Tanks are very micro intensive. You have to siege and unsiege them to get somewhere after all. And during fights you have to unsiege and move forward with individual tanks and not all of them. If your Siege Tanks are caught unsieged your entire army can be gone within seconds.
Personally I play Zerg because I like the mechanics, and would say the game is balanced pretty good. There are games where I'm being dominated the whole time, and others where I dominate my oponent. Scouting and good macro helps a great deal, but there is always a bit of luck involved too.