TMDB doesn’t seem to work very well either…
-
For now I am more or less not using Plex metadata anymore. Instead I use TinyMediaManager with it’s IMDb plugin for both Movies and TV Shows:
https://www.tinymediamanager.org/
This puts .nfo files in each Movie and TV Show folder, which contains the metadata. -
Then I use the XMBC nfo importers, to get the content of the .nfo files into Plex:
Metadata agents for exported XBMC library
https://github.com/gboudreau/XBMCnfoMoviesImporter.bundle
https://github.com/gboudreau/XBMCnfoTVImporter.bundle
I got perfect IMDb ratings now 
A couple of crappy things to note, which means manual maintenance is now a real and regular thing:
-
Doesn’t look like Plex automatically imports changed nfo files, like it does changed media / subtitles files.
Any TMM updated media items needs manually triggering “refresh metadata” for each one…
-
For TV shows, the XMBC nfo importer plugin incorrectly uses the “dateadded” field in the TMM exported nfo file, as the “first aired / premiered” field… Which is very unfortunate. This means if you add a 30 year old show like “Friends” today, it will get todays date even though it’s from the 90s…
The result is basically that if you just run it on the entire library, all TV Show premier dates will be wildly inaccurate
https://github.com/gboudreau/XBMCnfoTVImporter.bundle/issues/106
So. Before I (manually) run TMM on a TV show, I manually edit the show in Plex and lock the date field it has found from other metadata agents.
Movies are ok as they have a longer date field with month and day. TV Shows only get the year in TMM, which the XMBC nfo importer ignores. -
There might be an extra step here as TVDB has a bug where “first aired” date is no longer set on all shows. Which means Plex can’t match the show against TVDB and it is necessary to (manually) run the “match” process in Plex…

https://forums.thetvdb.com/viewtopic.php?t=51614
Has been an issue for at least a year, so I doubt a fix is imminent.
This Kodi plugin has implemented a workaround for TVDB’s missing year bug:
Thetvdb : Show year issue
Using TMDB agent for TV shows instead of TVDB might be a workaround. -
TMM does not have automation but needs to be run manually, every time you add new content or want to keep IMDb ratings updated

It is probably possible to write a metadata agent which can run a local install of TMM when adding new data, but I do not intend to do this. -
TMM is pretty good at auto matching stuff when you run it first time on the entire library, but may need some items corrected. The first import manually checking every match for correctness is fairly tedious.
It might be possible to write a script which grabs TMDB / TVDB ids from Plex’ database to get more accurate matches, but I haven’t checked if there is a way to feed this info to TMM. -
When updating IMDb ratings with TMM on a library it already knows, it sometimes matches your existing media items wrong

There is no option to only update data, the routine is called “search and scrape” and cannot be split. It updates the entire data each time, when it is really only the critic rating field I want.
I am not checking every item each time as that would drive me nuts. I do fix the wrong matches as I stumble upon them in Plex. Not ideal.
All in all. This is a pretty crappy workaround…
However. Metadata is now correct, apart from the odd wrong match from TMM.
No more IMDb ratings at 7 which should be 4. And no more premiere dates which are 30 years off. I can choose and sort based on air dates and IMDb ratings again. Which is great! 
But. What used to be a pretty awesome Plex setup is now painful with lots of manual overhead. Needless to say, the IMDb ratings are a must have for me. This was also the reason I left NetFlix.
I have started looking into Emby to see if that is able to provide correct data, without constant need for manual maintenance which I find painful and a silly use of my time.
Paying for Plex feels wrong at this point. If nothing else, it looks like Emby staff actually replies to their users. Which must feel really nice.