StoreMi, like FuzeDrive or Intel's RST or Storage Spaces, etc. is a tiering solution. PrimoCache, DrivePool (w/SSD), etc. are caching solutions. There's a difference between the two. With tiering, the faster SSD tier gets the "hottest" or most-accessed data while the slower HDD tier acts like cold storage. There's some caching here with Storage Spaces for example (1GB by default, for small writes) but in general that's how it works, much like a SSHD. With caching, all incoming writes go first to the SSD and then are later collated and written sequentially out to the HDD. SSHDs also do this since they have DRAM write cache, but you get the idea. Optane for its part is a bit different, if Optane-accelerated that is, it combines everything into one volume that acts as both caching and tiering, but I digress. There's certainly advantages to doing it if your SSD is small and HDD large or if you have specific needs, but if your SSD is sufficient for OS + apps then it's safer to manage manually.
Me? I use Storage Stages with striped SSDs and HDDs for temporary workspace. I personally would not use it (or StoreMI) for an OS SSD + HDD solution...