I'm sorry, I don't. I'm kinda locked into my niche and don't consume much of the wider cybersecurity industry or have a handle on who would be a trusted resource outside of my particular realm in application security and vulnerability research.
For at-least some insight, I can recommend https://www.youtube.com/@cwinfosec its a pretty small channel, but he has some great "Interview with a ..." content. I enjoyed his interview with Alh4zr3d on red teaming experience. Most of the interviews are more offensive security focused, but he has a few different jobs that he's interviewed and can give some exposure to the type of work being done.
Microsoft's Security Response Center has also started a podcast called The BlueHat Podcast I haven't listened to a ton of it yet but they seem to have a decent variety of professionals on talking about stuff which can potentially be a source.
Namespaces basically are a sort of kernel enforced isolation. A processes enters a namespace and to that process it might be root on its own machine. Behind the scenes the kernel is kinda translating everything it does so into its own little sandboxed area instead of the root system. But inside that namespaces it legitimately thinks it is the root user and can exercise most of the functional that is only exposed to privileged users. (f course the kernel limits what it can do to only being inside it's own little space so that alone isn't an issue.
When it comes to hardening, the namespaces are not inherently insecure. The difference is in the "attack surface" an unprivileged user has access to through them.
A simple example of this is mounting a filesystem. Now the user won't be able to like remount a privileged filesystem or something it'll be isolated. But let's say there is a vulnerability in the exact filesystem code in the kernel, your server doesn't mount any exfat drives and you disallow automounting of anything for hardening. So even if the issue exists an attacker couldn't exploit it because the exfat code isn't reachable as normal user. With a user namespaces though a user becomes root of their own little area so they can actually ask the kernel to mount something inside their namespace. So now with a namespace an attacker can get access to exploit their theoretical exfat filesystem vulnerability.
tl;dr the problem with having namespaces on is it allows unprivileged users access to a lot more "potentially" vulnerable code that could be exploitable.