I didnt say that the Tullys are the most powerful faction. It is logical and in lore that Tyrells be more powerful.
I just said that IF you beat the siege of Rivverun (meaning both Jaime and Tywin armies destroyed) which is quite difficult and then you ally with Tyrell you are done. Eating the remaining Lanns with the Reach is easy, and Joffrey will not attack because Stannis serves as a buff zone. Then just move everything east and step by step kill the Vale.
So one faction which is supposed to be screwed can with a single victory be one of the most powerful and with chance of victory.
THAT is the balancing issue.
PS: I love having TW discussions in a rather irrelevant forum