I just took PDD earlier today. I found that it was pretty solidly SS (my weakest point) with some BS, and CD-level questions relating to both of those. There were a lot of multi-answer questions (fill-in-the-blanks, hotspots, drag-and-drops, 'pick three'). The test itself was still unbelievably laggy which is especially annoying at the case studies, it's really almost discouraging to use them. Still, I wouldn't expect NCARB to fix this because why would they.
I didn't really know what to study beforehand, and I had heard it was similar to PPD, so I studied far and wide. I wish I had been a bit more focused and studied some certain things more in-depth, but I didn't really know that while preparing--the NCARB prep material is pretty vague. It was similar to but more specific than PPD; there were some big SS topics which I felt didn't even get covered at all, but instead PDD (or my version at least) seemed to pick a couple of systems/topics and go deeper than I had expected. So anyway I'm happy I get the free re-test because I'm probably going to need it.