Especially for photography in my opinion you want the SSD speed for the system processes and a fast second drive for current and temporary/scratch disk files, etc. I understand that people here disagree with this and will repeat the "Apple didn't intend it this way so it's not necessary".
This manual placement issue is largely overblown.
Users don't generally control how HFS+ uses its cache of storage contents in RAM. Users don't generally control how storage drives use their cache (RAM in both SSDs and HDDs). They don't control how SSDs do they mapping of which blocks contain which pieces of data.
This isn't really about "intent". If folks could actually point to mainstream, not quirky corner cases, where the caching heuristics of CoreStorage were bad over long term then would be a point. Mostly though this is about control and time shifting longer disk copies to different points in time.
Yes and no. The applications will not be on the SSD necessarily. Especially once things start to fill up a bit stuff will be all over the place with no control over any of it.
At a precision level of control? No. However, no control at all? Also no. If an app hasn't been used in 2 months then yeah it may have been shifted to the HDD. However, how possibly time critical could this app be if hasn't been used in 2 months. If use once every month and it takes 1 minute longer to load then have saved yourself all of 6 minutes over the course of 6 months. That is versus not having some photo reference maybe every week or so not on the SSD. The notion that people can more accurately track which of 100's or 1,000's of files actually got used and how often over the span of 4-6 weeks as well as a computer is a bit off. There is not of much empirical evidence that works well over the long run.
If the active working set if bigger than 128GB - OS/App size then Fusion presents a problem. However, for a 30GB OS/App size that's about 90GB of "elbow room". A mariginally larger workset size of 128GB would work if the SSD component was 200GB in size ( could partition down a 256GB drive if going to be writing very intensively ).
If one of the two drive fails you're out of business (only until fixed and your backup moved back of course).
If have data on only one drive and that one drive you are out of business too. The need for back-ups is the same.
The difference is in the length of time to restore from back up. It isn't necessarily a question of data loss.