I used Miri for some key deno libraries and spent a fair bit of time cleaning up the violations. Many of them were real unsoundness bugs due to reference aliasing.

Unsafe code absolutely needs Miri if the code paths are testable. If not all code is Miri-compatible, it's worth restructuring it so you can Miri test as much as possible.

Note that Miri, Valgrid and the LLVM sanitizers all compliment each other and it's really worth adding all of them to a project if you can.