IMHO, the outcome of an EtG match depends on the following factors:
90% on the decks facing off
9% on the RNG
1% on player skill (as in how to play given deck with given draw order)
Of course, the deck design is also pure skill question, but in a given match-up (such as in war) its already pre-determined.
Since the generals are all very experienced, plus the team effort chimes in designing the decks, the overall effect is that the decks end up being on par with each other at least on the long run of a war (individual matches of course happen where one deck is simply superior to the other). So, good ol' RNG becomes the deciding factor at the end.