I'm probably just biased, but to me the problem with modern C++ is its complexity. The mental model of the language, its syntax, and the amount of history / number of interfaces a programmer needs to be familiar with to be productive are all too large. Rust isn't any more complex, but it's not radically simpler either. Carbon doesn't look like it solves this problem. It would seem that creating a language that is expressive, performant, and small while still general purpose is quite a challenge.
The Github Page[1] says this: "The best way to address these problems is to avoid inheriting the legacy of C or C++ directly, and instead start with solid language foundations like modern generics system, modular code organization, and consistent, simple syntax."