I personally find that for a lot of queries, especially ones of the "trivial" or "smart" kind where I'm looking for some old meme or bit of fandom trivia, DDG often returns results that are noticeably worse. Not much worse, and not always - but often enough that if I don't find something on DDG, I frequently try the same search on Google anyway.

That said, DDG isn't that much worse than Google in terms of search quality, and is perfectly fine for more "serious" queries - It's an inferior user experience, admittedly, but I'm willing to drink RC Cola instead of drinking cane-sugar old-recipe Coke that also is contaminated with lead.