Not enough memory channels
The majority of CPUs do not have sufficient memory channels to make good use of SCM. Without being able to dedicate a memory channel to SCM, it is relegated to fast NVMe storage. (The AMD EPYC range is one of the few that can afford to sacrifice memory channels to SCM. One SCM memory channel would reduce the maximum DRAM on EPYC from 2 TB to 1.75TB per CPU.)
Another problem with SCM is that it is only effective for local storage (SCM and CPUs on a single motherboard) - serving the data on SCM over any sort of network negates its advantages. For a cloud server this implies that jobs using SCM cannot be easily migrated to another node.