It is as tough as Fordyce. I think it is tougher than the Rubicon primarily because it is such a long trail with few places to relax. Although close in actual mileage, it takes much longer to run it and the Dusy has a huge elevation change compared to the other two trails. Also, there is no Rubicon Springs or other oasis in the middle in which to hunker down while making repairs. In terms of overall difficulty with all factors considered I would rank these the "Top 3 Must-Do Trails of Central and Northern California" with relatively equal overall difficulty. The same but different if that makes sense.
Tight trees on the first day like to take out tops and windshields. A long wheelbase is not your friend here, although a buddy in my former jeep club, the West Coast 4WD Club in Orange County, once did this section with a CJ-8 Scrambler pulling a M100 Jeep trailer. Body damage is common for wider vehicles.
Steep, narrow switchbacks up Thompson Hill will remind you of Cadillac Hill, but it is much longer and much steeper a climb.
Once you reach the top you will experience rocks, mud, lakes, more rocks, meadows, another lake, more rocks and if you are lucky some good fishing. The views and scenery are spectacular.
The Gatekeeper at the north end illustrates why the Dusy has a reputation for being tough, and is the best opportunity for the obligatory poser shot on the trail.
One review suggests that Clawhammer at Johnson Valley would be similar in difficulty to the toughest sections of the Dusy. Sounds about right in terms of difficulty scale for the toughest obstacles on the Dusy, although they really are very different trails and I would not characterize the total trail difficulty of the Dusy as the same.
One of the issues I am hearing is how portions of the trail are being chewed up by larger rigs on very large tires (think JKU's on 40's) with the result that the northern 1/3 of the trail has become more difficult. I can see how this would happen because the trail has become more popular but was designed to be run by narrow, short wheelbase CJ's and FJ40's and such rather than a bunch of angry bird grill wide ass behemoths. You will have to let us know if this is true.