If you mean the "Quick" assistant, I only use it for light queries but I think it's pretty accurate usually, especially if the search results have the answer somewhere. It sometimes takes your query too literally, like when using quotes it will search with quotes, find nothing, and search again with quotes. Which is silly because if I wanted literal search, I wouldn't turn on the assistant!

But since I'm on the Ultimate plan, for stuff that matters I usually use Fable with a custom assistant (prompt), and sometimes the stock "Research" assistant. Both work really great for me. I use the "Research" assistant for more technical questions, it tends to dig up information from forums and cross-reference it with the source-code file it digs up from github and quote the RFC that it has directly fetched too.

Though I usually prefer my custom assistant prompt, which Kagi seems to combine with their general prompt in the background. This works really, freaking great most of the time. My custom prompt is an elaborate version of "answer the question, but then try to second-guess what the user is doing and also provide a different perspective that could be more useful than a direct answer". This has derailed my narrow train of thoughts a few times and turned me around into the right direction.

That said, I haven't played around with my own local agents for search yet to compare (in part because Kagi does a very good job at this), and this is something I want to try, in a "own your tools" sense. But the first thing I'll make sure that it uses the Kagi API for its search.