The problem isn't that `is-a` is hard, it's the divergence of `is-subtype-of` and `is-subclass-of`. Take a look here, I think it explains it rather well.
http://okmij.org/ftp/Computation/Subtyping/
I would argue that nominative subtyping is natural (basically sub-classing), since it matches our ability to assign arbitrary meaning and relationships to words, which is incredibly useful when having a conversation. Ya, maybe a set isn't exactly a bag, but close enough. Birds mostly fly, but penguins don't, we can deal with that.
But the main problem with subtyping, nominative or structural, is how it messes up Hindley-Damas-Milner style type inference. But that shouldn't be surprising that an FP theory for type inference wouldn't work well for OOP.
