I think that many people forget about the incredible mobility that heroes are capable of. In our campaign, the Rebels would often try to outgun the Imperial forces set against them with predictable results. Remember that when you take a move action, you gain movement points to spend over the entirety of your activation. That makes it possible to hit and run.
At the end of our campaign my Rebel players were fearsomely equipped, but when they didn't move they died. The part that makes this tough is how some of the missions are so high stakes, like the Hero Upgrade ones, that learning curve can really take its toll.
Yes, but no matter how manoeuvrable there are if there's a squad of stormies between you and a door you need to open or an objective you need to interact with then you have to kill the stormies first. If the Empire keeps putting baddies in your way you run out of activations.
By the time that the Imps can start cranking out low cost units due to the threat level, the Rebs should be geared / skilled up well enough to be able to handle that. There's no accounting for the dice, of course. And I can see how a cruel Imp player could hoard agenda cards and use them very effectively. But having played a few campaigns, the Rebels can absolutely win and they actually do win as wel.