I say let's way until release. If anyone here has concerns, they can just wait a few weeks after release for reviews and gameplay footage. Given the high profile nature of this game, I'm confident every aspect of it will be analyed ad nauseum. None of the speculation here is helpful unless your only goal is to convince people how horribly misguided they are in looking forward to it.
I would imagine the balance issues stem from the nature of the mechs themselves. I used to play a bit of mp in MW3 & 4 and I don't remember there being more than a handful of mechs on either side irregardless of map size, but that could have been due technical limitations. Until we see real time combat within Titanfall, there is no way to quantify how combat will actually work or whether the balance concerns are justified.
I think it's perfectly reasonable to compare this game to the likes of Battlefield and Call of Duty, but the nature of the combat means the scale isn't 1:1. The map might have stategic points of interest that trivialize or stalemate beyond a certain number of combatents.
This is complete conjecture of course. The truth is no one really knows. We only know the basics.