> They are "better" in that they are, in fact, more constrained; only when the error case arises will there be any accessible error value; otherwise, the actual expected value will be found. Since go uses an ad hoc product type, you always get an error value and the return value, even if they are mutually exclusive most of the time.
But multiple return values are there for much more than returned result and error. You conflate that with the specific use of returning result and error, and based on that, you claim that sum types are better. That's a straw man par excellence.
Sum types and multiple return values are not mutually exclusive, though several languages today use tuples to emulate multiple return types (see Scala, Rust for examples).
I am not a language designer, but I have become interested in languages in the past couple of years. Sum types require some sort of generics implementation, which Go does not have. I think the design choices the authors made regarding the language have made adding generics that much harder, that now they are struggling to find the "Go way" of fitting them into the language.
But multiple return values are there for much more than returned result and error. You conflate that with the specific use of returning result and error, and based on that, you claim that sum types are better. That's a straw man par excellence.