> everything is represented by a bitstring, a function shouldn't care what the name is of the bitstring passed to it, it should just work if it has the correct bits.

;)

Nominal type system can be built on top of structural type system with zero runtime overhead, but not vice versa (you'll have to add tags, which will take additional memory space).

The problem with nominal type systems, is that it needs to support parametrized types, otherwise it's hard to impossible to write reusable / generic code.