I've written a new version that calls watchexec by translating cargo watch's options and defaults, so given that watchexec has several issues of ours covered, technically these are fixed:
It's a bit of a cheat, though.
You can install with this lengthy command:
$ cargo install --git https://github.com/passcod/cargo-watch --branch just-wrap-watchexec
And if you don't have it already, you'll also need to $ cargo install watchexec.
This is not published to crates.io because I haven't completely given up on making my non-wrappy version work, so if I publish further non-wrappy releases they'll go on crates.io on the 4.x release boat.