The disadvantage is that there is more data stored in total: e.g., in MatrixStore typically 2.4x original data size length, vs an equivalent 1.5x in similar-levels-of-protection n+m algorithms. However, we believe the many other advantages MI+RAID6 provides (as outlined in this FAQ) plus the relative inexpensiveness of hard disk space compared to other factors (less node locations/CPUs required for example) make MI+RAID6 the most appropriate protection for Nearline and long-term archive data.
Furthermore, in some configurations MI+RAID6 will have the same overhead as PO-EC n+m. E.g., where data is replicated to a geographically dispersed location then each location in MatrixStore will kept at 1.2x original data size, whereas with EC, typically each location will kept with a similar level of overhead.
MatrixStore also offers the option to select how many instances of data to keep.