Here's a significant difference, in the course of that comparison:
In JRRT's setting, when Morgoth tortures and corrupts elves, their physical appearance changes.
On the input side: elves are "the fairest creatures in Arda". The Quenya word "Vanyar", translated as "fair", refers to their light-coloured hair. When their eye color is mentioned, it's gray. Some Elves have brown hair, but JRRT wrote, in these words, "no Elf had absolute black hair".
On the output side: we've already quoted physical descriptions and established which human populations match those descriptions.
Meanwhile, in the 'Verse, the process which tortures and corrupts ordinary humans into Reavers changes their minds, and NOT their physical appearance. In the episode "Bushwacked", we see a person on both ends of the process. I could have played that character, because in the 'Verse, an ordinary humans can have black hair, and as a Reaver will still have black hair.
I could not play an elf whom Morgoth corrupts into an orc, because no Elf ever has my hair color. I could only play the output side of that process. Apparently the process which turns a good person into a bad person, also makes a fair-haired person look... more like me? Well, that explains a lot, doesn't it?