That is a ridiculous statement. In the case of userns exploits there have been many and it means that every unprivileged user can obtain root on the machine.

Whereas rootful docker is a well known thing, run on millions of machines, and none of the vulnerabilities discovered in its entire existence is as bad as any single priv escalation issue caused by allowing unprivileged users to create a user namespace.

Lol that's just like saying having no door is better because some people can pick locks and occasionally open doors.