Just tried to upgrade to MC19. Live TV did not work after upgrade (no signal error). I inexplicably had to Run As Administrator once, and then it worked even after subsequent non-administrator runs.
Still, even though I imported my MC18 library including settings, there was a ton of work for me to re-do. None of the MCE remote buttons to get to Guide/Recorded TV/etc. worked anymore. None of my recorded TV rules were migrated. And that just resulted in me again realizing that there is no way to actually view and manage series subscriptions in Theater View - 3 years after MC started doing TV and there's still no way to do this. I just don't know what to say. Anyway I couldn't identify any specific thing I'd gain from upgrading so I just decided to save myself the work and go back to 18, despite the fact that I've already bought an MC19 license. Sorry.