While some of the codices are clear winners compared to other the elimination of allies helps tilt the table back closer to level. Is it a balanced system no‚ but generally closer than previous iterations. If the rumoured 7th just tweaked really a handful of rules vehicles overwatch the psychic powers and addressed upper limits on rerolls. Then all that would be left is to FAQ some of the really stupid shit like serpent shields. 40k is never going to be the game we want it to be until Tom Kirby isn't relying on it to keep him rich. If they'd just put more effort into polish and tighter products then GW would be miles ahead.
The lack of structure in force organization prevalent in 6th Ed. is a glaring factor that highlights the unbalanced nature of 40k, but it is not the only factor. Vehicles have been a massive problem to balance in the game for many editions now, and they constantly are swinging back and forth between being ridiculously good and utter crap. 6th edition has brought the advent of Flyers to the game, which further unbalances the game. In a nutshell, 40k is just an elaborate game of "Rocks, Paper, Scissors", but not all armies have equal access to scissors, if any at all.
No, because GW has already bitten to far into that notion that allies are a thing you need. I play Sisters of Battle and my only option for dedicated anti-air would be the stupid immobile pop-gun and twin-linked multi-meltas on my Immolators; people do not allow Forgeworld regularly enough to make the investment of the Avenger Strike Fighter enough.
Then you lose fluffy allies like Wolves + IG or Chaos United.
My local tournament scene is comped to hell and back. I'm relatively new and asked what's the deal? Usually comp is a death knell for tournaments, but the TOs (store managers) said that when running uncomped events, the turnouts are much lower. The use Swedish style comp and limit any form of attachment to the primary army to one "thing" be it Inquistion, Allies, Formations, etc.
I don't think straight 5th edition style would get the job done in competitive 40k right now. As a dark angels player, I don't think I could go toe to toe with an eldar or tau list without tailoring and if I do come with a hard ravenwing list a heldrake or two ends the game before it begins. The only thing that would get me back into a tournament setting for 40k would be to eliminate battle brothers, making those allies of convenience and I'd be okay with reasonable forgeworld units.