As iocage pretty much only wraps FreeBSD system tasks, I can't see why a language different than "shell scripting" would be an improvement here - does it use API calls instead?

I have no idea how iocage is being rewritten, but if I were to rewrite iocage in Go I would call the relevant system calls directly. I would not wrap exiting programs or shell scripts.

The iocage shell script was totally unmaintainable. I know because I forked it and used it for my own purposes, until I stopped and wrote my own thing (coincidentally, also in Go). Implementing state machines correctly in shell script is painful.