Please correct me if I'm wrong, but doesn't nushell suffer from the same problem as powershell that all the nice fancy stuff works only for in-process commands and external programs are bit of a second-class citizens?

To me interesting question is that is it even possible to build rich strucure-aware shell-like cli environment that would allow seamless integration of external polyglot programs in the same vein as unix shells where only minimal amount of stuff is required to be builtins (/bin/[ is cute example).

It is a difficult problem, at minimum probably requires all compatible software to be explicitly made so. But for best experience even the basic unix fundamental of stdin/stdout/stderr+argv/env might need re-evaluation.