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But that would work only when you have 2 factions facing each other. Once you introduce the possibility of facing multiple factions on each side the logic behind a faction benchmark breaks. And this mostly would apply to 1v1 only.
You can have OH as a benchmark and balance around the allied factions against it, but have completely different results against OKW.
What do you do when you have a faction which is OP against OH but UP against OKW? What you do you do when that faction only works in 1v1 but is horrible on teamgames and viceversa. What about discrepancy on skill level as well?
Quite simply first you balance all allied faction vs Ostheer and then you adjust OKW accordingly.
Rather than a FACTION as a benchmark, you want a certain unit archetype to be the benchmark. Giving you the upper limit of how strong a type of unit can be.
Again what you use as benchmark is irrelevant as long as it constant and used for direct comparison.