The type system of Rust requires you to be clear about which data your code is touching, and whether that code can mutate it or is just reading. This allows a sufficiently smart task system—like that of Bevy—to automatically distribute work across threads for better hardware utilization.

Also, one of the main reasons I'm interested in Bevy is that the engine is modular, pluggable, and extensible at the core renderer level, which is nice for those of us experimenting with different rendering techniques. Unity in particular is not a good fit for this type of thing.