I think it is too early to tell how well balanced Galactic Assault really is. Right now you have a lot of people who just don't know what they are doing.
I've been in groups of 10+ people though and we straight destroyed the defending team because we were actually playing the objective.
Disorganized chaos favors the defenders. But if you as the team make strategic coordinated moves you only have to "win" one time while the defenders have to "not lose" the entire match.
Also, HvsV is by far the LEAST balanced format, at least right now.
The dark side have 4 top tier HvsV heroes (Vader, Kylo, Boba, Maul) and the 2 strongest characters to be targets in the game (Boba Fett and Darth Maul). Plus Vader who is absurdly over-stat'd. The dark side can bring an absolutely absurd amount of crowd control to the table.
The light side has 1 top tier HvsV hero... Luke...and they don't even have 4 characters that have an improved jump. They have absolutely no answer for Boba Fett who can hang out on ledges that are inaccessible to any other character. They have no one who can 1v1 Vader or Boba Fett if either of those characters are played correctly. They have no one who can catch Boba Fett or Darth Maul. I play a lot of Boba Fett and if I die in a match it's because I did something stupid. If I don't get an elimination every time my rocket barrage is off cool down, it's because I messed it up. Boba Fett is absurdly imbalanced and his rocket barrage cards need nerfed. The dark side has a distinct advantage on maps where there are ledges especially since non-force users are free kills anywhere near a ledge and while Luke and Yoda have pushes, they actually have to position to knock people off, while Maul and Vader have directional control on their position altering abilities... It's pretty ridiculous when the guy who pulls a team's back line into melee range and then freezes everyone in place and crushes everyone with a huge damage multi-swipe is the weakest character on your team...
If played seriously dark side should win every time. If you lose as dark side you either were dinking around with inferior heroes or you were way way way out-skilled.