I have experimented with ultrasound for 14 years now. To summarize it briefly, it works for me, but only if I catch DD early!
I use 1 MHz, at maximum strength, for between 2 and 5 minutes per session, several times a week. I treat for 5 minutes in 'active' areas, 2-3 minutes to maintain any old sites.

I have a severe case, classified as Dupuytren’s diathesis: early onset, mutiple fingers/hand areas affected, feet affected, family history.

I developed the first two nodules about 24 years ago and needed the first surgery on one in 2000. Then I discovered ultrasound and have treated the remaining old and subsequently any new areas with it, also the area with the scar from surgery. So far, I have avoided any recurrence in my first operated finger (according to my surgeon very unusual for someone like me, there is absolutely no sign yet) and have contained or reduced two other new areas, one for 14 years (start in 1999), the other for just a year now. All treated areas have shrunk to a small hard core, after initially softening and flattening and are not progressing, as far as I can tell.
But if I tried to treat any tissue that was already quite far advanced , then ultrasound failed. This was the case for the second 24 year old contracture (14 years old at the start of my ultrasound treatment), which I have more or less contained with radiotherapy (2000) followed by two needle aponeurotomies (2007&2012). My other failure was a lump which started 3 years ago as a painful 'cyst' in a finger, I simply did not recognize it as Dupuytren's. That lump was treated with ultrasound about 18-24 months later, but it failed and I just had to have an operation on it 4 months ago.

Another twist in my story: two weeks after my most recent surgery, three areas developed new nodules, two next to the scar and one in the other hand! Coincidence or triggered by the operation? I don't know. All of these I am now treating every day for 5 minutes and it seems as if they are getting softer, flatter and smaller. If this continues, I will be convinced that ultrasound works for me!