both sides are quite balanced, but the game right now is slightly in favour of the Rebels. The Rebels change their dice results by directly modifying the dice itself, but the Imperials seem to tackle this problem by adding more dice. Hence, Rebel strategies tend to look more stable. Also, Rebels rely more on actual hp, whereas Imperials rely more on the evade dice, which can make things somewhat random / unstable at times. Moreover, Rebel strategies tend to be more focused on firepower, while Imperial strategies are focused around agility, flying around and dodging arcs, overwhelming with numbers. Since the attack dice is stronger than the defense dice, we can see where this is going. Also probably the main reason why you see people complaining about the TIEadvanced and not the Xwing
Imperial ships also tend to be a lot more maneuverable than Rebel ships, but that difference is negated by the Rebel's access to turret weapons and ions. Imperials do have access to mines and ions, but their ions tend to be stuck to large ships (and hence a lot more tricky to use), and mines seem to be more of a hard counter against other Imperials and do not do anything really particularly amazing against the Rebels (as compared to Rebel turret vs Imperials)
I would agree with this.
The game is slightly in favor of Rebels because of the above, and the fact that Rebel ships are just harder to take down. When you add up total HP (with shields counting as HP) across a squadron at 100 points, it's pretty clear where the advantage lies as well.