One major point over ssh is that: "WireGuard securely encapsulates IP packets over UDP." - so you avoid all the issues of tcp-over-tcp;wireguard is a "real" VPN.
sshuttle doesn't pass TCP over TCP, it does some work on the "local" side before sending it over TCP so it's actually just "data-over-TCP"[1].
[1]: https://sshuttle.readthedocs.io/en/stable/how-it-works.html
But still doesn’t do udp or ip, only let’s you connect out (not in, which is often desirable but still a limitation).
Also, you lose information about where connections originate - to the recipient, it all looks like it came from the sshuttle host.
One of the most obvious features is that you get roaming with WireGuard (like Mosh) which I don't think you can get from sshuttle (it might be technically possible to add, but I don't think it supported it last time I used it). It also allows for management of the VPN interface like a regular interface (so you can set iptables rules and other complicated network setups using it), rather than relying purely on proxying. And you don't need to give people SSH access in order to use it.