My understanding of his method from a friend who did it recently is you read the material before you go and then he concentrates on hammering home the pertinent points for the exam rather than trying to teach you a subject in 5hrs.
I can't speak for Derek Davidson, but I did my RT in the way described above locally & it worked very well.
I don't know of this particular groundschool, but I think it would be irresponsible to think that you can just do a days study for each exam and be a confident and safe pilot afterwards.
There is so much information to learn; I would imagine this is more of a brush up class after you've studied to help you get through the exam, but I could be wrong.
What I found with the RT was that by being on my own with the tutor the delivery of the subject matter was very much personalised, but that the tutor put everything clearly into context. I'd say that as a learning experience I took far more away from the few hours one-to-one that I have been able to use in the air than I actually used in the exam. To be honest, stopping to take the exam was a bit of a dissapointment given the amount of teaching & interaction that I had with the tutor, it made the whole experience very enjoyable.
People learn in different styles & speeds (Mrs Batninth teaches adults so I have this drummed into me) and the style that I see DD advertising can work very effectively.