Soundness bugs come up once in a while, so yeah there is doubts. I wouldn't claim it to be sound until there is at least a proof of soundness of a theoretical model of it (which you can then show the implementation matches)
There is actually a CS student working on it... from the University of Washington I think? Check out rust-redex. I don't know what the status is though - it doesn't seem to have been updated in a while.
Anyway the devs have said that whilst a 1.0 release will mean backwards compatibility for language features, they will fix any soundness bugs that come to light.
