ETA: this is based on the open source code which may be modified, userland daemon status, /dev/tap usage... Portability... etc.
In any case - tinc is excellent
I should have not mentioned anything about other software. It is irrelevant to the question.
The question was/is: How portable is WireGuard?
There exist closed source third party clients for Windows/OS X, and wireguard itself has currently two (unreleased) reference libraries in Go and Rust to support future cross-platform clients.
So the answer to "how portable" is probably "portable" :-) (I don't know if there are any plans for _kernel_ mode implementations for anything but Linux, but cross-platform user mode clients are part of the project's goals)