The issue is not that user namespaces cannot be used for secure isolation -- the problem is that it has been used for privilege escalation in the past. It definitely is more secure than it was 5+ years ago and there are ways of restricting its use on running systems through a couple of sysctls (in addition to the out-of-tree patch that Debian and Arch use).

But, in the case of running things in containers, you can stop exploits of user namespaces through seccomp filters that block unshare(CLONE_NEWUSER) -- Docker does this by default.