Gosh, maybe I wasn't paying attention -- but we took 21 from Stanley to Boise a couple of years ago, and it seemed like a nice piece of cake. Beautiful scenery, and a good surface. Yes, there were some hills and curves, but nothing to write home about.
But I would defer to folks (unlike me) who have made that drive more than once, so you should take my advice with a very large grain of salt.
And I took 75 from Stanley to Challis (not recently -- in 1981!), so my impression of that road is out of date -- but at that time, it followed a river and was not difficult at all.
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)
Our trips -- pix and textAbout our trailer"A journey of a thousand miles begins with a single list."