The borrow checker rejects loads of sound programs - just read https://rust-unofficial.github.io/too-many-lists/

Aliasing rules can also be problematic in some circumstances (but also beneficial for compiler optimisations).

And the orphan rule is also quite restrictive for adapting imported types, if you're coming from an interpreted language.

https://loglog.games/blog/leaving-rust-gamedev/ sums up the main issues nicely tbh.