I think DDG is more of a legitimate competitor in the search space than other engines running off the Bing index. My understanding is that they've been pretty transparent about using Bing and Yandex as sources for its product, have done quite a bit of work to tailor the results they spit back, and maintain their own knowledge graph for features like instant answers. They seem to function more as a metasearch engine.
From ehat I've seen, search engines like Ecosia and Qwant aren't very clear about what they bring to the table, beyond serving as a proxy for Bing that doesn't store as much data on you.
I don't think there's anything explicitly wrong with that. But I don't think they really merit anybody's attention either. Especially since they don't explicitly advertise what they are.