I used Kagi for a few years and was completely satisfied.

I switched to Ecosia because I want to support European alternatives, but that's all. I tried Qwant for a while, and for some reason it was frustrating me (I assume that the results were not satisfying for me?). I wonder if Qwant is better for search in French? Not sure. Ecosia hasn't been frustrating though, and I cannot say that it's worse than my experience with Kagi. I just wish I could pay Ecosia instead of having ads (which my browser seems to block anyway).

Anyway, I always wonder why it is that a search engine (say Kagi, Ecosia) can "work well" and another one (in my case Qwant) can be "frustrating". If they all tap into Google/Bing and others, I would expect similar quality? I'd be curious if someone could explain!