Haskellers and Rust fans think they own sum types, and people read their comments and blog posts and believe them, and decide they don't want sum types because they don't want to go down the horrifying Hindley-Milner rabbit hole.

But meanwhile it's just perfectly idiomatic Erlang and Elixir, none of that baggage required. (In fact, the sum types are vastly more powerful than in the ML lineage - they're open.)