By a huge margin the most frequent time I drop back to Google is because they have unit conversions built in. So I can send duckduckgo a query like "!g speed of light / 400nm in terahertz" and I get the calculated unit-correct answer immediately.

It's basically replaced my TI-89 for years, I hate doing calculations with units by hand and it's super error prone to not include them in the calculations.

For everything else, duckduckgo has consistently provided a suitable result to answer my question for 90%+ of stuff, no worse than Google. It's super rare for me to check if Google has better results for normal things. I see them as a different ordering rather than superior ordering, so I think of Google more as an alternative than a fallback. They clearly both interpret whether a site is a good match using different algorithms but I don't know I'd say one is strictly superior.