One thing is setup of jump hosts (basic version just being a box with sshd enabled + users with authorized_keys, which is beyond simple), but it's the limitation of hosts. I have 1 big network where a set of users are only allowed to jump to 1 or more unique host each, with myself and a few others being able to jump to every single one. I believe that your solution requires things to be on different subnets to isolate them, with separate jump hosts for each subnet, correct? Here, I only need one sshmux for everyone.
Another "fun" feature is that sshmux can also throw unknown users to a separate host, such as ssh-chat, for support or hanging out.
Actually, in case of -oProxyCommand="ssh -W %h:%p jump_host", you're still fully secure in the case that the jump host (sorry for not calling it a jumpbox) is compromised. This is true both for normal SSH servers and for sshmux. They can mess with the original raw ssh connection that requests the forward, but this will just screw the connection up. They can connect it to a wrong host, but that will provide incorrect host keys. The agent isn't forwarded, so they don't get to sign anything with the private key. Messing with the new connection that gets established is subject to SSH's normal MITM resistance. This means that even root on the jump host would require real SSH protocol vulnerabilities to attack the connection.
Good luck with Userify!