There are quite a few "rare" invasions that almost never seem to happen anymore.
Most of the Scarlet Gorge invasions are rare simply because very few folks ever want to be in that zone.
Any Fire invasions in Scarlet Gorge
Any Fire invasions in Gloamwood
Certain Death invasions in Scarwood Reach
Certain Air invasions in Shimmersand
Certain Death invasions in Freemarch
Certain Earth invasions in Drougtlands
The list goes on. They are all nearly on existent on my server. And to be honest, im fairly sure the reason is the continued spawning of "event invasions" taking their place or screwing up the RNG factor. A great example is "The Blight" invasions which are 90% of what you see in a few zones these days, especially gloamwood. The waves of madness are crazy commonplace as well. And dont even get me started on the old death invasions from the RoS event.
Quite simply anyone going for achievements is having to get lucky as hell to knock them out.
DEVS MUST LOOK INTO THIS! REMOVE OLD EVENT INVASIONS PLEASE!