The borrow-checker is a big source of frustration for beginners, because it forces you to write code differently, so it takes a lot of getting used to. But I don't think I would characterize it as a source of complexity in the language. Frankly, once you get used to it, it makes the language simpler, because it means you don't need to worry about shared mutability. The complexity that does exist tends to come from the zero-cost no-copy abstractions which rust is so fond of, or `unsafe`, which exposes you to a whole world of complexity which safe rust protects you from.