MT/R's are good on everything but mud and wet rocks. For some reason they don't like to grip wet rocks.
Sipe em for the wet rocks & they work better, but they will chunk out easier on the dry stuff.
I love my MT/R's except for the constant barking when on slick rocks and going around corners on the street. I've never really tested them in thick/sticky mud but they seem to do better than my old BFG's in all area's other than wear. They do wear better than Interco's though on daily driver.