Nim has more features like generics and metaprogramming via templates and macros. You don't have to use them, but if you read someone else's code that uses it, you might have trouble understanding it, which probably doesn't happen with Go.
And to be perfectly transparent, I know relatively little of Go. My impression is that nim has more features in general, which could be considered to make it more complex. That can be a good and/or bad thing depending on perspective.