It seems to me that it's not their utility, but lack of support in general for the sorts of changes that enable its wider use. E.g., looks like it's getting practical use in FreeBSD: https://www.cl.cam.ac.uk/research/security/capsicum/freebsd....

Linux has seccomp, but I think that was changing the access for an entire process. The language-focused aspect seems useful to me, from that application aspect where maybe I want access to something, but I don't want to pass that access on to all the code that I might call from a library.