Yes, i agree but the two projects are vastly different. I agree the red tape would of likely been the hardest part 
Unfortunately we do not have access to either projects documentation so we can only speculate on how they designed each product, we do not know the architecture of each product. They are vastly different games and it really depends on how each was coded. I think Gearbox mentioned that it could be on the cards for the future, so that’s promising.
We then have the problem with peripherals. Rocket League works well with all of them and there is no distinct advantage, if anything a controller is probably better due to use of analog movement over four way for movement. The right stick however, is perfectly fine for camera control as you do not have to precisely aim etc. A FPS is an entirely different concept. Aiming with a 6400 dpi mouse is, highly likely, much more accurate and faster than with a controllers right stick with its sensitivity at max. This could cause balance issues as well as an unhappy community, it depends on how they handled it.
Rocket League is also, highly likely, less demanding on systems due to its concept and therefore different framerates/hardware are likely not a huge problem. Battleborn has alot more going on on screen at once and and is an entirely different game, so someone with a higher frame rate is likely to have an advantage with a shooter/melee pvp game.
I’m in agreement that I would be interested in cross platform play
. I just think that it would be harder and take far more work than a universal account
. They are entirely different features with different requirements and workloads.