The big problem isn't invalid UTF-8 but invalid UTF-16 (on Windows et al). AIUI Go had nasty bugs around this (https://github.com/golang/go/issues/59971) until it recently adopted WTF-8, an encoding that was actually invented for Rust's OsStr.

WTF-8 has some inconvenient properties. Concatenating two strings requires special handling. Rust's opaque types can patch over this but I bet Go's WTF-8 handling exposes some unintuitive behavior.

There is a desire to add a normal string API to OsStr but the details aren't settled. For example: should it be possible to split an OsStr on an OsStr needle? This can be implemented but it'd require switching to OMG-WTF-8 (https://rust-lang.github.io/rfcs/2295-os-str-pattern.html), an encoding with even more special cases. (I've thrown my own hat into this ring with OsStr::slice_encoded_bytes().)

The current state is pretty sad yeah. If you're OK with losing portability you can use the OsStrExt extension traits.