I think in the upstream kernel LSMs are also still the only way to prevent a process from creating child namespaces where it has privileges?

E.g. if you can cat CAP_NET_ADMIN even within a restricted namespace, you have access to huge amounts of horrbly broken kernel code. It's easy (for people who know how to exploit kernel bugs) to escalate privileges from there.

Distros have their own fixes for this issue so namespaces definitely aren't useless in practice for sandboxing. But the basic mechanism just doesn't that well suited to it.