Rendering / text layout & especially display & positioning of images & in general interpretation of CSS used to be "alpha level" IMHO the last time I've tried to read epub book (around 2018.
).
And like most open source Linux software it screamed "UI designed by programmers / developers". On paper with its list of features it looks absolutely marvelous but in reality using it always felt clunky.
To me even Kobo's firmware at the time of original Kobo Aura ( that is the only Kobo device I ever used) looked and worked in that "pre-beta at best" state (even that font boldness and size change feature I always considered a huge plus for Kobo was buggy and the text size changed with practically each book I've opened) and KOReader was worse (more "opensource-y and alpha) and the same impression I get with most other UIs used by smaller companies that sell eink readers whenever I watch a YouTube demo video.
Just couple of days ago I've opened KOReader on my PW3 and let it upgrade from some version from 2019. to the latest stable version from 2022 and took a look on what's changed and was somewhat more satisfied with text layout... until I got back to a KFX book opened in stock Amazon reader
Even if all other things were equal between two reader programs the difference in quality of available dictionaries (the ones available for KOReader are terrible) would always take me back to using Kindle stock reader. Quality of English-English and English-Serbian dictionaries is important to me as english is not my native language but most of the books I read are in english