One thing to keep in mind when comparing DuckDuckGo to Google is that people do not use Google with an alternative backup in mind. When you DDG something and it fails, you can always switch to google.

But what about when Google fails? Unlike DDG, there is no culture of switching between search engines when googling. Typically, you'll just rewrite the query for google. And as rewriting the query is an entrenched part of googling, you are less likely to notice this as a failure. It is this training that's the core advantage nostrademons points out.