I have a theory about Nunyunuwi. The big drop off of spawns happened after patch 2.35. One of the major changes in that patch was the change to spirit bonding. It's possible that spirit bonding gear is the key to spawning him. Since most people that have been trying to spawn him in big battles and such would be in gear that can't bind in the overworld, it would lead to less sightings overall. With the random spawns being attributable to low level players questing in gear that easily binds in ST.

I'm going to test this out tonight before the maintenance to see if I can get him to spawn. At this point anything is worth trying.