There is no much complication. Several providers which give multiple static IPs. Client expects the packet back from same ip and port to where it sent its packet (this is specially needed if client is behind some firewall). The mangling rules I tried from a post here http://forum.mikrotik.com/t/wireguard-multi-wan-policy-routing/174145/90 where there is a hack with adding clients src to an address list to be used for detecting the outgoing interface and src IP (something like a conntrack). But this does not work from that post, as the wireguard src ip is set before mangling and pref source is not used. Nor a srcnat to wan IP does not work. as it do not preserve the src port. If I srcnat with src port, then it does not work at all.
The initial discussion post from where I relate is here http://forum.mikrotik.com/t/routeros-blatantly-ignores-pref-src-can-this-really-be-a-bug/180360/66