macOS High Sierra, Messages won't search at all
I finally got a solution for this in the Apple forums.  Still not sure why it worked but, here's what was suggested:


  In Sys Prefs>Spotlight>Search Results, uncheck "Mail &amp; Messages" (and leave it unchecked), then
  add your drive to Spotlight's exclusion pane, wait, then remove the
  drive from the exclusion pane, wait for Spotlight to rebuild it's
  index.


I had to leave the machine overnight but this morning, Messages search works again.