Structural types are one of those things that annoy me in both theory and practice, when I have to touch Go code.

In theory, it's annoying because it prevents you from expressing constraints (think: monad laws, or Rust's Sync/Send traits). In practice, it makes stuff like go-to-definition nearly useless, which makes it a pain to read code written by other people. Especially when combined with the almost sadistic module system.