Anyone have any experience migrating Plex server install from Windows to UnRaid environments?

Theres not really much to migrate.  Make sure you are using Trakt to keep up with your playstates, then perform a fresh install on unraid.  It will redowload all the metadata, but thats no big deal.

I did this a few months ago yes. It was literally a matter of copying over my old plex install folder and following this guide https://support.plex.tv/hc/en-us/articles/201370363-Move-an-Install-to-Another-System. A caveat though, if you want the new server to appear as the old one (so you don't have to reinvite your shared users etc) then you need to copy over your server identifiers from the Windows registry to the unRAID install. I can't remember the exact procedure, but it's something to bear in mind.