I used it for a bit and I must say that it is has great UX and UI, especially for beginners.
All features you could want already integrated, so no “jank” set up required like with Anki.
The big reason that made me crawl back to Anki, however, was Yomichan.
Kitsun kinda sucked for me as I use monolingual cards for quite some time now, and (at least when I used to use it) there was no easy/integrated way to create automatic J-J definitions for your cards (it only had a Jisho integration and the more you understand a language, at some point you kind of realize that most translations just really suck/are not clearly defined compared to monolingual definitions, especially in regards to nuance and usage hints).
Using a clipboard hooker with Yomichan is the most efficient way for me to quickly create monolingual cards outside of web sites, since it can also be combined with OCR from pictures and games (Game2Text e.g.) and sites like animebook.github.io makes it trivial to mine cards from anime subs). You can even use an epub-reader in your browser to quickly mine with Yomichan from that (ッツ Ebook Reader).
(@ Kitsun users, did they add some way to create monolingual cards easily?)
Another smaller thing keeping me in Anki now is the Migaku Kanji God add on that creates Kanji cards automatically whenever I see a vocab card with an unknown kanji (which even at 2.7k happens more often that I expected).
I learned to use Anki five years ago for when I was still in uni for Japanese studies, so I feel very comfortable navigating the not-so-cool looking interface, but I have to admit that if you are completely new to SRS systems/software, it takes quite the effort and time to create (and actually understand!) a set-up inside Anki that achieves feature parity with a pre-made solution such as Kitsun.io.
tl;dr
Kitsun is great, especially for beginners, but at some point there might be a wall that will limit you in your vocab mining.
Anki on the other hand is rather confusing at first, but can grow with your needs and modded to your individual needs + it’s free and open source software, if you care about that.