The set of problems for which Go is the best programming language to use is empty, simply because Rust exists. Rust has non-nullability, immutable shared state...
Rust is nowhere near ready. A language is more than a language spec and a compiler. Its the standard library and ecosystem of tools that make a language truly useful. Go has those, Rust does not. It is unfair to both projects to compare them.
Given _any_ language, I can find some non-empty problem set where that language performs badly (or is ill-suited). That does not take anything away from the language. There is no "perfect" language yet; maybe we'll get one in 5 years... [1]
[1] http://www.great-quotes.com/quote/63358