The point I was trying to make is that if you have to rely on runtime type information, you might as well use sum types in a more strongly typed language, where the scope of the code gets limited to the type itself.
With "duck typing", you need to have an extra test for types which your code is not supposed to work with, whereas with sum types, the type checker will help you verify that it cannot happen.
That being said, there are situations which a strong type system will not be able to model.
Anyway, at the end of the day, what matters is that you (and your coworkers) feel comfortable with your code. I know by experience that it's not always the case.
To expand on another commenters example, suppose you have a sum type that models successful computation with a return value or a failure with an error message.
If you have tags / constructors, that's all easy. But if you just use naked unions, you can not write code that deals with error messages (or strings in general) in the success case.
With "duck typing", you need to have an extra test for types which your code is not supposed to work with, whereas with sum types, the type checker will help you verify that it cannot happen.
That being said, there are situations which a strong type system will not be able to model.
Anyway, at the end of the day, what matters is that you (and your coworkers) feel comfortable with your code. I know by experience that it's not always the case.