At Joyent, we deployed public-facing multitenant workloads based on zones (and before that, jails) for many years. We seriously trusted it -- and had serious customers who seriously depended on it. So, now you know someone!
Give them some time to get Rust above that.
Very big ocean indeed.
You can look at the recent history of Linux kernel LPEs --- there has been sort of a renaissance because of mobile devices --- and count all the ways any shared-kernel multitenant system would have broken down. At the end of the day, it's not so much about predicting whether your system can get owned up (it can), so much as: "what do I need to do when there is a kernel LPE announced on my platform". If you're doing shared-kernel isolation, the right answer to that question is usually "fire drill". It's not a noodley thought-leadership kind of question; it's a simple, practical concern.