The project is meant to ensure that you can allow multiple users access through a jump-host style mechanism, while not permitting any other "abuse" of the jump host. You can lock down SSH a lot, but not as much as sshmux does. Security wise, the code is very, very simple and easy to follow, and even if it went rogue, that's no different than the trust you put in your usual SSH server. If this is a concern, do not use the agent forwarding mode, which would render a rogue server a pointless and unfruitful prank.
If you use the ProxyCommand mode, the only thing an "evil" sshmux would be able to do would be to break the connection. It won't be able to fake the endpoint if it is already in your known_hosts, as it does not have the real endpoints private host key. It is, therefore, secure in this mode.
The project is meant to ensure that you can allow multiple users access through a jump-host style mechanism, while not permitting any other "abuse" of the jump host. You can lock down SSH a lot, but not as much as sshmux does. Security wise, the code is very, very simple and easy to follow, and even if it went rogue, that's no different than the trust you put in your usual SSH server. If this is a concern, do not use the agent forwarding mode, which would render a rogue server a pointless and unfruitful prank.
If you use the ProxyCommand mode, the only thing an "evil" sshmux would be able to do would be to break the connection. It won't be able to fake the endpoint if it is already in your known_hosts, as it does not have the real endpoints private host key. It is, therefore, secure in this mode.