For the downsides, it requires linking a payment card so it's impossible for them to guarantee privacy (cryptocurrencies could save the day here) and it completely relies on other search engines like Google and Bing for search results which is a questionable long-term strategy in my eyes.
Got a quote on that? I was pretty convinced they (Kagi) did their own indexing, and results are significantly different from both Google/Bing.
https://kagi.com/faq#Where-are-your-results-coming-from
I meant that they were "completely reliant" on these APIs in terms of utility, not that 100% of search results are external. I'd love to be proven wrong on this but my impression is that without Google, Bing, and other APIs Kagi wouldn't be a functional and competitive product.