Right now the one thing more than any other that the cause of the persistent imbalance is the GC alignment. For the first time, Frontlines gave us content that we could have been able to go right into without waiting on a certain distribution of classes to fill- potential that was squandered as a result of the GC requirement. Changing that restriction (rather than adding another layer of restrictions on top of it) is all it would take to do away with the one-sidedess that so many are talking about. This imbalance is something that was present all along, but teams could overcome it with strategy and skill. However, what Slaughter does is take any imbalance between teams and amplifies it by 1000x, with bonuses to power and survivability being awarded to the whichever team proves to already have the most power and survivability at the various DPS race encounters against nodes.
If you take the player base for any game, and arbitrarily segregate them into three groups, naturally the average amount of skill and dedication will not be exactly the same in each of those three groups. At the time most people made the decision what GC to join, there was no relation to PVP. And someone making the choice now as to which side they want to be on would most likely choose the stronger, more coordinated group if they care about PVP wins, even if it means waiting longer for a match... and someone who doesn't care at all about winning and just wants to quickly reap the abundance of PVE rewards as fast as possible so they can upgrade their relics to raid with and buy crafting mats can join a weaker group, and make it even weaker.
Why would a player who actually cares about succeeding at PVP mode choose to continue to queue up alongside the same group of players, and against the same opposition, in a situation where they lose match after match due to being tied to a weaker alliance. What's that quote about how the definition of idiocy is doing the same thing over and over and expecting different results? Many of the people I know who played the Frontlines mode (the 72 person Secure mode that was effectively removed from the game) before and enjoyed it have quit out of disgust. What FL has become, if you're not in the dominant GC on your server, is a place for a lot of people who don't care about PVP to come get quick PVE rewards at the expense of a few people who try really hard to overcome that but can't. Here's how much they don't care in my GC, a lot of them don't even bother to commence! Whenever the counter is x players short of 24 or 72, I know that's how many players Twin Adders will be short when the match starts because maybe they didn't want to stop crafting or they preferred to chase another S-rank.