Namespaces look dangerous to me because they break lot of assumptions software was built on before. For example, sudo relies on /etc/sudoers being accessible only to root. But with unprivileged containers one can easily create a filesystem namespace where /etc/sudoers would contain arbitrary data. I think, SUID bit won't work in container, but there might be other ways to confuse privileged software using containers. Or not?

Also, if the container has access to dbus, one can try to exploit multiple services listening on dbus.