Why did I get an email from MacRumors alerting me to a post that was made Oct. 20, 2018?
It looks like they solved the problem for at least stock Fusion drives. Regardless, I highly recommend never using a Fusion drive. I would also not recommend a spindle drive inside. While I find APFS a much better file system for my purposes, There's documented evidence that spinners are not as fast with APFS. Also SoftRAID has little current ability with APFS. So I keep most of my storage on an internal 20TB Raid5 and a 24TB multi bay external enclosure. (a five disk Raid5 gives me read speeds of 450MB/s, triple normal spinner speeds) All my bootable drives or clones are APFS. I used to be able to use RAIDs with Mojave, but not anymore. In one situation I use a WD USB3 8TB as a 3rd backup and a residence for backup clones. The APFS system is much more convenient to delete an APFS volume or create a new one with two clicks. I never assign a size to the volume, and so all those storage drive volumes share all the empty space to all the other volumes in that APFS container. If I actually have to boot from the slow external clone, it can use all the empty space on the drive (for system files and a OS scratch disk) not just what's on the volume. The really bad part of that is that my cMP can only boot from an original USB2 port, not my USB3.1 gen2 ports. However, those drives are so slow, it doesn't make much difference. For fast external boots, I have to use eSATA3 to boot externally. That doesn't work too well for an iMac...