There were definitely a few versions of releases 19.x that had buffering issues with larger 4K videos where it would just stop playback, but the latest ones for versions 20.x seem stable for me. Only issue I can think of is sometimes the touch interface on a tablet gets confused and won’t let me scroll up or down, but restarting the app clears it. No issues for me with the pc app interface using mouse pointer and scrollwheel, or the remote controlled interface on a FireTV.
edit: after reading about jellyfin, the main drawback I can see to using that is you have to install specific server-side software for it (unless I misunderstood) while Kodi is self-contained on the client side and just reads any accessible folder locally or on the network. (Not bashing jellyfin as I’ve never installed or used it, just noting that difference.)