Unobvious. Not hard. To the chasm that is getting someone to pay for search, getting them to install an app and follow tedious but simple configuration instructions is a gap in the sidewalk.
This is a clever workaround by Kagi, but a glaring hole in the Safari extension API surface area.
Eventually I gave up and uninstalled their extension. I switched to using StopTheMadness to do the redirects instead, and am having much better luck. I did switch from redirecting Google to redirecting Ecosia at the same time, and this might be the difference, and while I'd fully agree that Safari doesn't make it easy, but I think the base problem is that their browser extension just doesn't work that well.
(If you are familiar with both, you will understand that switching _to_ StopTheMadness for a better interface is pretty high in irony!)
When all it should've been is a "custom search engine" option like Firefox does.
Calling it "unobvious" is PR newspeak for jumping through the hoops to set up a Rube Goldberg machine to do a basic search.
There was a period of time when they had two apps, and I agree the old one was stupidly complicated. The new one, Kagi for Search, doesn't require this.
Like, should Apple have an open API for routing searches? Maybe. Would that get abused? Probably. Do I think Kagi should be on Apple's list? Yes. Does prioritising a 50,000-user engine into iOS's defaults create other issues? Yes as well.