What are the SEO challenges associated with private search engines, like DuckDuckGo?

[searchencrypt]

I read recently that DuckDuckGo doubled in size in 2017. With their search engine, and other alternatives to Google, taking part of the search market away, how can SEO/Marketing/Web pros keep their websites optimized and get traffic from these private search engines?

(Also, do any of you have experience with this? What portion of your search traffic is coming from private search engines?)