Good.
Ok, if desert region is required to recruit White Mumakils than it will be very unlikely to see this unit in game. Appreciated. I guess Mordor, Isengard, OomM, OoG would have serious problems to fight them.
Maybe make them AOR Units for Imlandris, but with no stronger stats as usual Eldars. Otherwise, the Elves become too powerful.
Make them weaker and cheaper. That is better imho. It would be a weaker unit than cave trolls, but cheaper, or give them more Trolls per unit. Let's say instead of 12 Troll per unit (huge unit size) give them 36 Trolls (maybe smaller body size if possible). Something like a lower tier unit.
Maybe a unit for OoG. I think they are from that region. Maybe a middle tier unit, instead of a super unit. Could be with Ap, a dwarven killer unit, but weak against cavalry.
Ozz wrote that he wants to overhaule the small mumakils, because they are still to large and have Harad signs. I guess he just changed the Harad bowmen with Mordor bowmen. Therefore I am not sure if they are a good idea. Mumakils alone are a pain in the ass.
But, if you want implement this unit, than it would be better is the unit would be like a charge cavalry unit and not like a super archer unit like harad Mumakils.
Will say the Mordor small Mumakils might be less a problem, if they would not invulnerable against anything except Javelins, catapults and Balistas. They should be more like a heavy cavalry unit with similar stats like Swan Knights, imho.