Thank you for this perspective. One concern I had along these lines is that the lrcat and lrdata previews files appear to the unaided eye as one file. But they are not really one file if one reveals contents. I know nothing of how the OS would handle these components in the instance of Fusion SSD/HD technology. Is there any concern that the catalog and previews components are not kept together in their native "package", some straddling the SSD/HD boundary, and that we in turn loose efficiency we might gain if the Catalog and Previews lived, in total, on a SSD?
I don't know whether Fusion operates at the file level or the block level. I suspect the latter, which would mean it could split files across the two disks without incident.
I don't think there would be any performance issue with having the catalog on the SSD and the previews on the hard drive, though if you were regularly viewing folders the previews would likely be on the SSD.
Really, though, I don't think it's worth too much hand-wringing about. There are two things I'd state in broad terms:
1) Fusion drives are an excellent mix of speed and capacity, at a low price. Let them do their thing, and you'll be pleasantly surprised. Just because Photoshop users are used to endless meddling about scratch disks, etc., doesn't mean it's not a waste of time when dealing with Lightroom, and things like Fusion drive :-) Everyone worries about getting 100% of possible performance gain, when in fact if you leave things alone and do nothing, Apple has a solution that gives you 99% of it without the need to think about one single aspect of it.
2) You should always have a good backup plan. Time Machine would work well, and in the case of a house fire, an offsite backup like BackBlaze or Crash Plan Pro is good as well.