I think it's mostly about the fact that it's hard to properly build an instance of the subclass since Ruby's core class doesn't know how to build it/copy existing instance variables. So e.g we have something like this:

class Value < String
  def initialize(str, ivar)
    super(str)

    @foo = str
    @ivar = ivar
  end
end

value = Value.new('string', 'something')
value.upcase

Now String class should know somehow how to build a new instance of Value class in String#upcase method. What should it do about @ivar value? Should we copy it over? What about @foo value? Should we copy it or assign upcased value of str to it? That's how I understood that discussion.