So, by disjoint unions, I'm going to assume you mean sum types, like a value that can be either a pointer or an error, but never both, and you use special keywords to access one or the other.
Sure, that's really useful - you guarantee that you can never access the unset half of the value. But it adds a lot of complexity, too. You need a way to declare values of this type - this means more keywords and/or more special syntax. You need a way to then access both halves of the values, so now you have to add pattern matching... that's actually a lot of complexity to add to a language that only has 25 keywords.
All that, instead of just doing what Go already does, which is to return two values and just have a convention of checking the error before the value... which works really well 99% of the time, and doesn't require all the rest of that complexity.