The docker project has done quite a bit of work on container security, so I'm not sure it's entirely fair to say that them not enabling user namespace by default is for that reason.

For example the work that was done on their seccomp filtering and apparmor profiles.

My guess would be that it's as User namespacing can introduce some issues (e.g. where mounting host filesystems), that they've decided the trade-offs aren't worth it.

Also looking at the CRI-O trello (https://trello.com/c/Ak2yMcpf/714-epic-cri-o-support-for-use...) it seems like user namespaces isn't even an option there yet?