The main difference I see between the two tales is that the king of the mountains's accursed people are a singular (mostly irrepeatable in itself) and temporary case - though your comparison makes for a vey interesting reading
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[As a side note, the case of the forgotten people seems rather problematic to me: did Isildur indeed have the power to alter the design of other Men, in a rather significant manner? Only the valar could do that, Imo, and they thoroughly avoid it.]