So, Star Wars: Rebellion has an inherent flaw. It is almost impossible to gain control of a well fortified planet without literally blasting it to pieces. I experienced this aspect just the other day, when trying to gain control of Commenor, an Imperial held planet with a dozen squadrons of TIE fighters and intercepters, as well as twelve squads of various imperial troops and a General Veers in command.
Let me explain some things to you about this situation. First, a planet blockaded with an enemy fleet, or with fighters is harder to run missions on (because units have trouble getting there). Secondly, every additional unit of troopers on a planet makes each mission harder to succeed at. The third problem, is having an enemy General on the planet, which increases the chances your mission will be detected and foiled. So, all three of these aspects make it extremely difficult to run a mission on the enemy planet. So, running missions on the planet to kill troops, destroy facilities, and capture the enemy general is just a waste of units.
So, what is left to do? Invade with a space force, and take decisive action by planetary bombardment or assault. The first problem, is taking out the opposing space force. At this time, I was running around with a few frigates and an escort carrier, with very few starfighters left operational. Once invading the system is complete, the planet is still sitting there as a fortress.
Planetary bombardment is often difficult. It has many problems, especially if the enemy has planetary shields and laser batteries. Your fleet is likely to be heavily damaged from the space battle, and you will loose ships if the enemy has a laser battery. Also, bombardment lessens your opinion in that system, and will cause neutral planets to join your enemy.
Alternatively, you can invade before bombardment. This means that the units you can carry with you will take on the entire enemy force. They have two disavantages: the enemy general and the defensive power of the planet. Both of these things make anything but an overwhelming invasion (2 or 3 times as many troops) impossible.
What's the point of this discourse? Simple, a nearly impenetrable base should not exist in any game. Difficult to crack, yes, but never so difficult to crack that it takes a significant part of your resources and actually hinders your ability to play the game. Let's face it, it's easier to blockade that planet, and any other fortresses, and wait for the units to be destroyed from lack of maintanance resources. There needs to be a way in to defeat the enemy.