Science, what else! 
Most people would define gameplay balance as being caused by the ruleset and its conditions, i possibly never will for a few reasons;
- Since i am a modder i must know what it means and can be done to alter it.
- The structure is already there and whatever choices someone else made is a direct consequence of carefully designed activity, results, situations that allow it to become 1)Challenging 2)Interesting 3)Entertaining and what else.
- That fragile concept of "Win or Lose" is the holy grail of programmers and as such, is an everafter quest for perfection or at times, a simple theory.
- Tech trees are the hypersonic vehicule that carries these principles into action, that also wrap your mind around a solution, that certainly define where most of your evaluations must be made.
If you believe that the researching aspect in a game is important, though.
We have a superbly linear progress towards specific steps to win, a pattern of thoughts to memorize, a series of decisions that lead to discovering WHAT it takes to balance YOUR own game.
Take a very good look at the following GTeChnos interface (status or release date pending compilation in Delphi or VBX05, btw), i'll find some correspondance between the 12 different tech-trees in Twilight of the Arnor while maintaining the usual Dark Avatar basic model & patterns... trust me.

In the meantime, get this absolutely essential tool by Christoph Nahr & Netriak; Galactopedia.
Cuz, technology is what you make of it or discover in alternatives provided by MODS!
6- PI, SP, GA, TG on surfaces