I've only had a few issues in 15 years or so of ownership.
The first Spartan broke down the first time out because the fuel tank was full of water and debris. Got home by rigging up some clean petrol into the carb. A few years later at Beltring in 2007 in the mud we lost steering in one direction, resolved by cleaning the cylinders and bleeding again.
My Sabre broke down at Beltring in 2011 due to a dodgy coil. Worked fine when it had cooled down, but embarrassing when doing a convoy round the site. I replaced the coil with one bought in the stalls.
The diesel Spartan was delivered at the farm and ran perfectly fine when thrashed around the fields etc. When I drove it home it died in about the same spot as the petrol Spartan, water and debris in the fuel tank. Got it home and on the drive with the help of an RB44. It also broke down in Hythe on a road run when the nut holding the ball joint for the throttle cable on came loose and bounced around the engine bay. We towed it back to the camp site with the Striker.
Scimitar drove into its first show in a smoke screen, resolved by backing off the handbrake bands a bit.
I've also been called out to rescue a Striker when the final drive locked up on a roundabout. We removed the track on that side and towed it home.
Loss of rubber on the wheels is also a common problem, I always take a spare anywhere we go, along with a recovery A frame.