I've mountain biked it twice. One time the rangers had to close it down for a few days from rain, so we were stuck in there, right at the bottom of Hardscrabble. It's pretty scary when wet - watched a couple of vehicles almost slide off. Also, Murphy's was a lot rougher then, tons of sand right below the top - really hard to make it over the last stretch. This has been a number of years ago.
Living in Moab, I've gone up and down Shaffer and Mineral Bottom a number of times. It seems like Schaffer is worse than it used to be. The switchbacks on MB were totally washed out a few years ago and they rebuilt it. Haven't been on it since then.