You can get stuck in Addo during wetter season. We saw plenty of big ruts in Addo when we visited in Feb (Summer) in 2015.
On that trip we did Mountain Zebra NP (just another 1.5 hr drive north of Addo) and Addo.
The road condition at Mountain Zebra was MUCH better than in Addo due to Mountain Zebra's roads are primarily gravel versus Addo's are primarily mud. So a big rain can make huge difference between the 2 parks.

Kruger's main roads are TARRED as they called it. The dirt roads were in poor condition during our visit in late Aug 2016 - very rutted and literally like a washboard. After a few attempts we decided NOT to drive our little Corolla on those roads because we were afraid the car could break down with the huge vibration the car experienced driving on the washboards.
Besides, there are no advantage for viewing animals from our 5 days experiences. The only worth viewing happened on the dirt road was 2 to 3 km from the entrance, of a Leopard. All our other viewings, including a group of lionesses, a lion couple mating, and a pride of lions, were viewed from the paved road towards the grassy patches. Rhinos were by the road side. Giraffes too. Elephants actually blocked our roads many times.. We also saw Impalas, Kudos, etc etc, galloped across the roads. Then a big wild dog pack was wandering on a secondary paved road very near Skukusa camp entrance. Given that, there is no need to wander to the dirt roads if the road conditions are poor.