I imagine a large part is just how one is used to doing stuff. Not being forced to be explicit about mutability and lifetimes allows a bunch of neat stuff that does not translate well to Rust, even if the desired thing in question might not be hard to do in another way. (but that other way might involve more copies / indirections, which users of manually-memory langs would (perhaps rightfully, perhaps pointlessly) desire to avoid if possible, but Rust users might just be comfortable with)

This separation is also why it is basically impossible to make apples-to-apples comparisons between languages.

Messy things I've hit (from ~5KLoC of Rust; I'm a Rust beginner, I primarily do C) are: cyclical references; a large structure that needs efficient single-threaded mutation while referenced from multiple places (i.e. must use some form of cell) at first, but needs to be sharable multithreaded after all the mutating is done; self-referential structures are roughly impossible to move around (namely, an object holding &-s to objects allocated by a bump allocator, movable around as a pair, but that's not a thing (without libraries that I couldn't figure out at least)); and refactoring mutability/lifetimes is also rather messy.