Problem installing High Sierra 10.13.4
FWIW I had huge problems running 10.13.2 thru 10.13.4. I'm on an Apple RAID that's HFS+ formatted. But apparently Apple suddenly dropped support for all RAID, even their own. Very annoying. Fusion is not a raid but it's kind of raid-like in many ways, I can see how that could cause the same problem I've been dealing with. The update runs through the whole process, everything seems to work, then it reboots and is still on the old OS. 

But here's how I finally had to upgrade my system: I had to clone my boot drive (the striped RAID) to a USB drive. Boot off that USB drive, and run all system software updates. They worked fine off a single, normal drive. Then I cloned back to my RAID volume. And obnoxious solution for sure, but it worked in my case, it may work for you.