- SSH uses TCP for transport, and tunneling TCP-over-TCP has considerable performance issues: http://sites.inka.de/~bigred/devel/tcp-tcp.html

- SSH doesn't support tunneling UDP out of the box (you need to either wrap UDP with TCP, or use SSH's -w option for creating a TUN interface, which requires root access).

WireGuard being a very simple UDP protocol has its advantages for things like this. The downside is you can't use onetun with the same private-key/assigned IP on multiple devices, since WG only supports 1 UDP endpoint per peer at a time.

Note: my tool doesn't support UDP in it's current state, but there's no technical reason it shouldn't be feasible (unlike SSH). I have a draft PR for it right now.