Inspired by Reinhard (it used to be Steve who was the first WeatherCatter in the world to upgrade his hardware with new software, but he is now deep into retirement, camping, road trips across America, having fun, talking to his wife, etc.), I took the plunge about 3 hours ago and upgraded to macOS 10.13.5 from 10.12.6. It went flawlessly, my fusion drive is still happy, and so am I. Apple announced today in the Keynote that APFS will be supported on fusion drives beginning with Mojave, coming this fall.
By the way, WeatherCat 3.01 ran 38 days without interruption until I shut it down for the os upgrade. It has now been purring like a cat for 90 minutes on the new setup.