• Solid axle is more scale for monster trucks
• Very few exposed moving parts, so they tend not to pick up sticks and rocks like an independent suspension (IS) rig
• They're more challenging to control at any given speed. That doesn't sound good, but it means they're as much fun to drive at 20 mph as an IS truck is at 30 or 40 --> don't need as much to run even a big one.
• Related to the last one, they're always just a bit out of control, due to bump steer, so it makes them feel faster than they are.
• On the negative side, they don't jump or take a hit as well as an IS truck.

It's counter-intuitive that they could be as much fun at lower speed, but it's true. Kind of like crawling can be fun even though it's mostly walking speed driving or slower.

Consider them for low or medium speed use and you'll be satisfied.

I have a Kyosho Foxx now; i want to add a (stock) Clodbuster at some point. I'll keep it stock and slow; they probably only go 10 mph.