Kurama considers himself as the strongest out of the bunch.
There is a difference between being the strongest, and flat out shafting the rest of the crowd...
I agree OP... I've never liked that. I mean you have Gyuki say, "tails do not matter," and Kurama pimp slaps him across the face. Then we fuse them together, and suddenly the ten tails is pimp slapping the world around in an incomplete state.... Which is to say, the 6 tailed beasts that were getting pimp slapped before while combining their power.... Just combined their power and are thusly pimp slapping Kurama.
The logic in Naruto is indeed flawed XD!
Not much we can say or do, but let's enjoy the brighter points.... At least it looks good (as far as being a boss goes). I guess Kishi edited Kurama's openness by making the Juubi OP...but that just becomes a spiraling tunnel of OP...
Still waiting on the Alliance to throw down something useful.