Not very. Most notably `env -S` doesn't work on some systems. I did get bitten by one script on Ubuntu using uutils where the script expected `uname -p` to work

Not very. Most notably `env -S` doesn't work on some systems. I did get bitten by one script on Ubuntu using uutils where the script expected `uname -p` to work but it just prints `unknown` (technically legal I guess). But when I went to fix it they already had. Probably was a bit premature for Ubuntu to enable it by default. Looking at the graph uutils will be fully compatible (or as close as makes no difference) in about 2 years, so I would have waited until then. Still, I think most of the push-back is just the usual anti-Rust luddites.