Plex is significantly different than XBMC. Plex is a media streaming/transcoding platform and organizer. XBMC (or Kodi) is a local media player and organizer.

A properly configured Kodi setup is far more flexible than Plex and offers a lot of significant advantages (PsuedoTV, for example) as well as a larger set of supported formats and playback options.

I use both pretty extensively and the idea of trying to replace either one with the other is distasteful. Different tools for different jobs.

Huh? Is it? Every time I tried Plex is was more limited and clunky when it came to media library organization. And it required me to install software on my NAS.

nope it isn't. Kodi is more customizable.

Plex is easy to install, but Kodi is much more advanced and powerful. Atleast it is for my fire stick and home network