If I remember correctly, even though Fuchsia is used in production, it is mainly targetting mobile or IoT devices. Nevertheless, the underlying micro-kernel, Zircon, is written in C/C++, which differs from Redox. Now, I’m not saying that Redox solves everything by writing the kernel in Rust. It will require plenty unsafe blocks to achieve what it needs, but it makes you aware beforehand that you should be careful about how you implement that bit of code. Having this clear marking could also make the kernel code review process more likely to catch issues.

Disregarding this, if I am not mistaken, Redox aims to be a drop-in replacement for Linux one day, both for desktop and server, while Fuchsia only wishes to be integrated in/replace Android. Linux is perfectly fine for most use cases, I am not suggesting otherwise! However, given how many issues resulted from overflow/memory corruption issues that could have been potentially easier to identify if Rust (or any other memory safe language) was used, you’d think that there is incentive to rely on it for kernel development. Linus himself made this decision as well when allowing Rust to be used in the Linux kernel development (albeit perhaps a bit too early).