Most C++ issues come from using every feature under the sun and accumulating loads of technical debt instead of picking one tool and sticking with it.
Which is the same thing Genode does[1], and it has served them well. Maybe a different language like ADA/SPARK or something else would have been better, but C++ was what they're familiar with, so they made it work. Although I like Rust, it's a bad argument here. Rust has been changing a lot and is still changing, and Fuchsia wasn't started yesterday. And with C++, you could even replace critical parts with formally verified LEAN[1], which has C++ code generation and should be even safer than Rust.