The idea of sieges coming up in Clone Wars makes me think the Empire can't just waltz in and squash ANY world they want. EU really helps flesh that notion out too a bit.
Well, in the Clone Wars you had two armies that were around the same size, strength, and technology level facing off against each other.(I think that it's due to Sidious wanting to keep the war going and the factions strength close to equal) The Separatists, especially during the Outer Rim Sieges, were digging in and fortifying their planets with the intent of withstanding Republic attacks. Therefore, Republic strikes turned into sieges spanning several months, like the battle of Saleucami.
When the Empire was in control and fighting the Rebels, they had superior technology and armed forces. To capture most planets, all they really needed to do was march a few AT-ATs in to destroy ground defenses, and send in Storm Troopers and other units to capture the major cities/towns. Even though the Empire may have had a hard time taking Kashyyk and Mon Calamari, they decimated Rebel bases on Yavin 4 (after the Death Star blew up) and Hoth (Okay, so the Rebels weren't really ready for that one.) If the Empire tried, I believe that they could take over any planet they wanted, either with fear, superior numbers, firepower, and/or technology on their side. Holding a planet, like Kashyyk, is a different story.