sw_interface_add_del_mac_address() is the one remaining place in interface.api that uses a u32 for the sw_if_index instead of the strong interface index type.

Aside from that, we have a primary MAC (this appears to be the l2_address of sw_interface_details, the response to sw_interface_get_mac_address, and set by sw_interface_set_mac_address) and also a list of secondaries (sw_interface_add_del_mac_address).

I would suggest that add_del_mac_address should be add_del_secondary{_}mac_address to make clear what it does, and we're missing a dumper for this information - it doesn't appear to be possible to list mac addresses, or at least it's not clear how you might do that.  Going a step further, we're inconsistent on object property names (l2_address, mac_address and mac).