Software defined software
"Run your instruction set on commodity hardware" is not a new idea, but "Software Defined $THING" has been a series buzzword rivaling "Cloud" and only outpaced by "IoT"... With very, very few exceptions, the hardware that runs storage arrays are good old x86/x64 powered instruction sets. They might have a custom Disk controller or maybe a custom ASIC, but they are x86 systems in the end. Specialized hardware adds a bit in a couple cases, but really, the software (and the feature set derived from it) and support from the storage vendor are what a customer is really selecting.
In my experience, the number one thing a customer is looking for from centralized storage is constant availability, then adequate performance, then the rest of the value added stuff. I would hold that building your storage in any sort of BYO manner will make delivering on "Constant availability" much less of a sure thing than a purpose built platform from just about any specialized storage vendor. Unless the SDS vendor is spending a lot of time building, testing, and expanding HCLS, and enforcing that tested/validated combinations of hardware are always used, BYO brings with it a lot of unnecessary risk. The BYO approach may have appeal to budget constrained shops, but, to be frank, budget constrained customers make for revenue constrained vendors.