I've been noticing the same pattern -- the results from DDG aren't quite complete enough. Sometimes, it's just missing a few minor things, other times, I'll run a search query and there will be a major search result that I know is missing, that I can get in one of the top 5 results from Google.
Which sucks, because I hands-down full-on no-buts prefer DDG's interface to Google.
I hope I'm not being uncool here, but a while back I privately suggested mining the data from "!g" queries as a way to see where DDG wasn't returning the results that people expected. Is there a reason you can't do that? In my case at least, I usually start with DDG, and if I don't get what I'm expecting, then I use a "!g" -- so you'd have a nice little database of results to perform comparisons from.