My biggest gripe with Rebels so far was that the main cast had only problems when they have to deal with big shots like Tarkin, Vader and now Thrawn.
Not to belittle them but the Empire's servants are often portraied as so .. inefficience that i my suspension of disbelive simply brakes. They would not have survived so far along if (my feeling by the show so far) 90%+ of the imperial military/navy are incompetent bunglers.
Yes, i am happy that Thrawn is back and portrayed like in the books, but again such a big name is needed.
The rebels operate at the same level as several smugglers/pirate groups and the Empire usually knows how to deal with those.
So far i don't get the feeling that the heroes earn most their successes.
Well, most PC groups only have big problems when a Nemesis appears. Groups of Minions and a few Rivals are usually just scenery to be chewed.