[duskwuff]

Usable, but only barely. As AXI / APB3 are intended for on-die communications, they lack the line coding and error correction used by other busses like PCIe or SATA. On the upside, though, they take a lot less area to implement.

[monocasa]

That's more a distinction between serial and parallel buses than on-chip/off chip. If you're not pumping a single signal to gbps speeds, you don't need the clock recovery and ECC that serdes style connections imply.

It all depends on your use case.