Just came back from a trail drive, which has a training obstacle course - side slopes, cross-axles, hill climbs, etc., all on very loose sandy/clay soil.
We put my mate's Defender through it in various modes, and there was very little to no spin in the correct modes. This is because we figured out that when you switch off the car, it DOESN'T remain in the Terrain Mode, but defaults back to Comfort, which then results in loads of wheelspin.
Thanks to PowerfulUK for their detailed video on this:
One more thing - this is software dependent, because unlike their video, our Defender defaulted back to Comfort mode regardless of which mode it was previously in when switched off!