It's a screwdriver vs toolbox situation. WG is a great simple screwdriver which provides encrypted tunnel between endpoints.
ZeroTier gives you fully configurable SDN with intelligent local/public routing, online interface, full nat workaround, custom addressing, global register requiring only a token to authenticate a node, and many other things.
I wished on more than 1 occasion that I could just use one of my ZeroTier nodes in other locations as an exit node and tunnel traffic through it though.
I think that might already be possible but it'd be nice if the configuration was abstracted away behind some simple interface.
That'd allow it to replace Wireguard and Mullvad for me as I mostly use it as a means to bypass georestrictions.
ZeroTier gives you fully configurable SDN with intelligent local/public routing, online interface, full nat workaround, custom addressing, global register requiring only a token to authenticate a node, and many other things.