Rebels have good units, just not a big variety at this point.
To make a decent rebel build costs half the price of a core set, without that cost barrier we would be seeing more rebel players and more rebel winners. That just leaves scum trailing far behind but the new scum content is amazing in terms of activation count and figure count.
Right now rebels have 13 distinct deployment cards to use, the new content will add another 6 and allow the use of a scum unit (10 choices). The number of rebel options has more than doubled once these are out.
We should also see new content announced at gencon and episode 7 tie in content at Christmas.
Then all 3 factions will start to balance out.
This year's OP is decided by play skill in an environment with limited figures/command cards/missions that is heavily skewed towards imperial.
Next year's OP will be decided by both play skill and squad building skill, with 3 competitive factions and lots of builds.
The skirmish game has a very promising future.