I have a DX513 attached to a DS1812+, filled with Seagate barracuda, Desktop, and NAS drives as well as some WD Reds. I have the two set to 2 disk redundancy, and have a total of 32 TB Usable space using 12 disks between the two (8+4). It works. Theres nothing more to say about it.
Simple Plug and Play design works well not really much more to say about it
Havent encountered any issues in 2+ years of heavy use