I have a dated opinion, it's probably still true, but compute power is much cheaper now.

If your player doesn't support the format, Plex will transcode it on the server -- which is extremely slow if all you have is just a junk pc as server, while on Kodi (or what I personally use, Infuse), you can use any file server, like smb, and they'll just play. No fuss, any lags will definitely be a network issue.