Actually its not really difficult if you take ur time. Lot of up/downhill turns at 10 mph on the Needles.
I would agree, it's not terribly difficult. The only Yellow route I haven't ridden was from Johnson's Siding north to Blackhawk. I believe this one is/was primarily gravel(?) which added to the difficulty.
Hwy 14 through Spearfish Canyon is a great ride, if not spoiled by the RVs.
I'm also a big proponent of the ride on Hwy 240 from I-90 Exit 131 to Wall, SD (Wall Drug) through the Badlands. Again, beautiful scenery and a great road! If you're coming from the East on I-90, well worth the detour. Stop at Wall Drug & have a milkshake.
****Wall Drug was my first employer out of High School and I continue to promote their business nearly 20 years later. Your mileage may vary, but I will always stop to say "HI" when in the neighborhood.****