I worked for a hosting company for 10ish years, and they started out their hosting by providing people with UML-based virtual machines for £10/month. In terms

I worked for a hosting company for 10ish years, and they started out their hosting by providing people with UML-based virtual machines for £10/month. In terms of isolation it is "pretty good". The guest has their own kernel, and you can run it as an ordinary user. Add a chroot jail and you can have 50-150 different UML instances running on a single host, each under their own UID. They can't talk to each other, and if they do manage to exploit the kernel they're only gonna get the access to the host which you restrict with chroot, and standard things. It's less of a risk to run isolated things than this than have lots of containers share the same kernel. An escape there means container-A can view/poke container-B AND the host. Of course there is overhead, and most people moved on from UML to Xen/KVM when they became available - both have better performance thanks to virtio, and both have more flexibility for networking, block devices & etc.