The Deisols (specifically the Mossy Core drop) is a good callout...the lowest drop rate of any item in the game is 12% (a little less than 1/8), however it is the LOWEST drop rate of any individual item needed for a quest...there are other drops which come from much less common enemies, but those drop rates are 25%, so maybe it just feels less like a bad drop rate and more of a slow hunt with the others...and, I wish I was in-depth enough to make the Deisols only show up on specific terrain types, haha...they are just programmed to show up in any "abandoned Eloin temple" style areas (long tunnel in Trabunco, Nanshe's temple, Yuramayo Caverns).
Apotheosis isn't out yet, but hopefully one day!