Q: Why are the UAD-2 SOLO, DUO, and QUAD PCIe cards incompatible with the Late 2013 Mac Pro?
A: The Late 2013 Mac Pro (black cylinder model) expanded the PCIe addressing scheme, making it possible to have up to 36 PCIe devices in your system (over Thunderbolt, of course). That means that PCIe cards used in that system need to support 36 Base Address Registers in order to be recognized.
The UAD-2 SOLO, DUO and QUAD cards use an earlier generation FPGA that supports 12 addresses, which was the previous standard implementation. The UAD-2 OCTO cards (as well as Apollo/Apollo 16, and both UAD-2 Satellites) are based on a later generation FPGA, which can support 36 addresses with compatible firmware. So it’s really as simple as that. Aside from the Late 2013 Mac Pro, the SOLO, DUO and QUAD cards are compatible with the rest of the shipping Macs and most PCs.