As a Kodi user, I must say it is very good on its core, and very bad on the addons side (which arguably is the part for which it gets recommended mostly)

It forces its limited model of text-based folders-with-files to everything. Also it's all Python, and I don't know if it's me but I always find quality issues first in Python projects than anything else. Error control is usually very lacking, and it's so frequent to see error pop-ups showing on here and there. You enter a menu and the first entry selected is ".." which is to go back to the previous menu (poor UX). All in all, Kodi for me has always been a player with good tech (it all basically works, surround sound, codecs, integration with hardware, etc), exposed as very amateurish UI experiences.