I think saying the game is completely balanced is a little wrong TBH.
You're saying that for all the maps, for all the different numbers of players, all of the playable characters are equally useful?
Think two rooks on crucible are going to lose? You tower farm the single lane up and you'd have to be a complete retard to lose after that.
Played a long game on Zikurat or Mandala before? How'd you go playing as QoT? Got caned by characters that rip you to shreds in the endgame like Reg? Hmm...I wonder what you could have done to stop that...not a whole lot because its a large map and the game is going to take longer, so you can't really stop the endgame from happening.
Think Sedna is awesome in a 5v5? Try playing one, your heal is far less useful because the amount of damage coming in is more than any damage mitigation strategy can handle, and you'll get nuked to death anytime you go near a group of opposing players (if you're on your own you're an idiot and if you're in your own group you'll get targeted)
On the other hand, in a 2v2 Sedna can negate a significant portion of the nuke damage coming in as well as being able to tank well enough to get in and pounce, silence enemies, making her one of the best choices.
So my point is...
Each character is good at taking advantage of certain situations that crop up within each game, and perhaps within the context of the entire set of maps and the entire set of character combinations the game really is well balanced, but the maps decide how often each of these situations occur, and as such different DGs are going to dominate different maps. For this reason I think there IS a need to talk about balancing concerns, and while there are many more OP/UP threads than necessary and many of them are promoting ridiculous discussion, that's one of the primary reasons for having the forum.
Thanks for reading and apologies if I sound like an ass.