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.
The souls of the undead (Forsaken, PC death knights, ghouls, etc.) are imperfectly attached to their bodies; the dark magic that sustains them is a buffer that prevents their souls from properly joining with their bodies. This is why undead feel only faint sensations of pain or discomfort from most physical stimuli, and why the Light is so painful to their existence. The primary exception to this rule are liches, as liches bind their souls to a phylactery and then use the phylactery to generate a physical form; this process is why lich bodies look nothing like their mortal bodies, and also why you have to destroy a lich’s phylactery to truly kill them.
It is difficult to say, as there are no known records of undead wielding the Holy Light before the Third War. There are reports, however, that some Forsaken have slowly experienced a sharpening of their dulled senses of touch, smell, etc., as well as an increase in the flashes of positive emotions that have otherwise become so rare since their fall into undeath. Unfortunately, this may be the cause of the Forsaken priesthood's increased attempts at self-destruction; regaining these senses would force the priests to smell their own rotting flesh, taste the decay in their mouths and throats, and even feel the maggots burrowing within their bodies.
Anyone that's not a Broken or Lost One it seems can use The Light due to it being mere willpower.
Benedictus himself stated: "there is only POWER!" He believed that Light was power and as such he could draw this power to it's full extent(even Velen hasn't used the Light in attack to that extent)!
Even a Lich can use the Light.
Infact it's probably much easier for them then it is for normal Undead since the can switch their Shadow connection with Light connection to the Phylactery.
A normal Undead probably would feel their rotting body at all times and be in pain if they replaced the badly done Shadow link between their bodies and souls with a complete link of Light(something a Light Lich could exploit when creating an army of Undead).