It depends on the definition of balance in this game for the DGs. Item balance is a different matter.
There two main definitions in this thread or for the game :
1. Balanced play is where every DG should have an equal chance of winning against an opponent DG.
2. Balanced play is where a tactic/build can be countered effectively.
For DG, definition 1 is not really appropriate. The game is not an individual death match.
Definition 2 is more appropriate, however it must be further defined.
In terms of DG balance the "countered effectively" needs to considered, an effective counter would be :
a) There are multiple forms of counter through varying mechanisms. Having only one specific counter would likely mean the tactic/build is not balanced.
Multiple forms of counter does not mean that EVERY DG needs to have access to that counter, that is the point of it being a team game.
c) A counter should ideally have no more cost than the tactic/build. Providing a counter where a DG has to be level 20, in order to beat a level 10 tactic is not balanced.
So using this we can theorycraft some scenarios mentioned :
5UB's - Their skills are easily countered, through Shields, Heals and Pots. Imagine 5 Spit UBs attacking 5 HP stacked Sednas. Not a pretty sight. 5 focused pounces and 10 focused Bishops. Spit DoT would not cut it, can be outhealed. I would say the Sednas would win every engagement, they can focus fire, whereas if the UBs were to try and interrupt heals etc, they would have to split their attacks.
An example, where a build is possibly unbalanced would be Minion Erebus. This build is not effectively countered at the moment without bringing substantially more cost to the table. It can kind of fit Definition 1, however, it is not really the argument that X DG cannot beat him. The argument is that the cost to counter, likely 2 DGs, far outweighs the cost to make the build.
I won't go into reasons, that is for another thread, but that would be my take on balancing for this game.
I would say that stats can fit well for item balancing, which is a slightly different topic. However, stats for DG win percentage and skill usage would have to used carefully. They can show where a DG or skill is woefully out of balance, however to truely determine the balance it would need thorough playtesting by expert players.