Input from a beginner
I would say the rovers have a very low center of gravity and there wide girth helps them be stable.
To me it looks like they could endure steeper angles than 25-30% and that restriction is probably torque related.
Here is a photo of a prototype called K-9 which shows how low the bulk of the weight of the rovers is and at over 600 pounds it looks like it would be quite hard to roll.
SteveS