True, but the customization would render your character able to use that without specific helms or gear, which is what people are going for, and also the story itself too. Being able to have a variety on character is lovely; though class locking might need to be a thing, I’m not sure. Especially for those with access to druids since I don’t think a dark ranger druid is possible. A paladin, however, would be, via:
" When undead use or are healed by the Holy Light, does it cause them any actual damage or harm, or does it only cause them pain (in addition to the intended effects of the spell)?
Channeling the Light in any way, or receiving healing from the Light, only causes pain. Forsaken priests do not disintegrate or explode from channeling the Light for an extended period of time… though they may wish they would."
" Can you please explain how “light” works? The lore states that undead are physically incapable of using the light, much like the Broken, but then we have Forsaken players casting healing spells, and Sir Zeliek in Naxxramas using pseudo-paladin abilities.
Without spoiling too much, we can tell you that wielding the Light is a matter of having willpower or faith in one’s own ability to do it. That’s why there are evil paladins (for example, the Scarlet Crusade and Arthas before he took up Frostmourne). For the undead (and Forsaken), this requires such a great deal of willpower that it is exceedingly rare, especially since it is self-destructive. When undead channel the Light, it feels (to them) as if their entire bodies are being consumed in righteous fire. Forsaken healed by the Light (whether the healer is Forsaken or not) are effectively cauterized by the effect: sure, the wound is healed, but the healing effect is cripplingly painful. Thus, Forsaken priests are beings of unwavering willpower; Forsaken (and death knight) tanks suffer nobly when they have priest and paladin healers in the group; and Sir Zeliek REALLY hates himself."
SOURCE: https://wow.gamepedia.com/Ask_CDev
Meaning any undead using the light would feel exceptionally searing pain, and if they rot(which some undead do not, it depends on hygiene and the undead themselves), their own bodies decaying, however with willpower they can grit and bear through it.
Druids, however, especially night elves, would clash, I reckon. Kul’tiran use a form of ‘death magic’ on that spectrum to shapeshift so they wouldn’t, but Night Elves definitely use life. And two opposing forces in one body explode (void and light), unless very specific to disc priests.