Planet side combat was certainly the weakest part. I always played Empire so would simply throw AT-ATs at the planet until it fell, which usually did not take much.
Yes it was, which is why I liked that Rebels could bypass it pretty well.
I would send in the Raid Fleets with 1 light tank unit, 1 heavy tank unit, and an artillery unit. Usually a couple of these was enough to almost completely cripple a planet's ground defenses, or at least take out their cannon so my fleet didn't have to worry about it. Made the space fights a lot easier.