What's the advantage of enroot over charliecloud, which is unprivileged in the sense of being installable in your home directory (given user namespaces)? https://hpc.github.io/charliecloud/

It is the same idea, we actually considered it at first. There are some differences in the implementation though and we built enroot with the idea of being more extensible. We also have a plugin for SLURM (https://github.com/NVIDIA/pyxis)

There are several things that can impact performance on "traditional" container runtimes. For example, cgroups, LSMs, seccomp (especially with spectre mitigations), network NS/bridges, etc. There are also more subtle things like being able to do CMA, or deal with shared memory. Most runtimes let you opt out but this becomes difficult to manage and secure with multiple users.