I can explain one of these. I’ve seen one about why Void Elves cannot be Demon Hunters.
“But they’re an elf!” I hear you tip-tapping on that keyboard. But I am gonna have to stop you there.
The fel and the void do not mix well together, if they mix the being could end up being torn apart.
The boss, Xhul’Horac in WoD is an example of this. If you pay attention to the lore in the journal…
"Summoned by Gul’dan from the depths of the Twisting Nether, Xhul’horac is one of the most powerful of the Void Revenants—a race of parasitic void-beings that tear holes through the planes of reality, consuming and absorbing the energies and beings within. Gul’dan has twisted Xhul’horac by feeding him overwhelming fel energy, and using his powers to tear open the portals through which he plans to rally the forces of the Burning Legion."
This is a being who is overwhelmed by being fed Fel Energies.
Now I can hear you tip-tapping again. “But Warlocks use both void and fel magic!” Slow down, get those pizza grease-covered fingers off your keyboard and buckle down buckaroo, I’m gonna yeet some lore into your face.
Warlocks manipulate Fel and Void, they are not beings laden with this power. It’s the difference between a paladin and a priest; a priest uses the Light as a tool, they study it and figure out their use, a paladin is a beacon of the Light, it courses through their veins (and I’m going off of the human/draenei/belf type of paladin here)
It’s also like a mage and a shaman. Mages study and use magic through formulas and manipulating arcane energies, Shamans are granted their powers and are beacons of the Elements themselves.
A Void Elf, being laden with so much void energy that it physically changes them, would be turned into chunky salsa if they decided to become a Demon Hunter because DHs are given demon’s blood to grant their powers.