Posits are a hardware-friendly implementation of unums, a sort of halfway house on the road to Gusafson's full vision of unums. Posits have a fixed size and are meant to be a drop-in replacement for IEEE floats. They can and are being implemented in hardware.
Unums are more ambitious. They would have variable lengths, so implementing them would require more changes than just changing the way a fixed-length set of bits is interpreted.
As part of the spec, a true posit implementation includes an exact accumulator unit. This has been described for decades, is pipelineable, and allows for, say 10e20+1-10e20 == 1. We've shown that this gives speedup and about 10^5 improvement in accuracy for linpack 1000.
Unums are more ambitious. They would have variable lengths, so implementing them would require more changes than just changing the way a fixed-length set of bits is interpreted.