I have ridden Mt Tam double once. In my opinion, it's not nearly as hard as Knoxville, which I have ridden 3 times. Any double can be hard, you have potential for heat, rain, cold, wind, digestion issues, etc. Some like DMD are always hard.
The time I rode Mt Tam I saw the sun only twice; on top of Mt Tam when we broke over the clouds, and at the finish as the sun was setting. It was foggy and damp the rest of the time. Next day my fingers were hurting from the constant shifting with all the rollers. Mt Tam and Coleman Valley are the two sustained climbs, Mt Tam climb is very early with fresh legs, and the ride is well supported. It's typical Marin riding, hardly a flat mile, but no real difficult climbs (well, except maybe Coleman Valley).