I just went through and methodically obtained all the quest starters for all the overland "dropped item" quests, in which you kill a particular mob until one coughs up a small box containing a quest starter for another zone. Most of these were fairly easily obtained, but I seemed to have a devil of a time getting a few. Before I /feedback it, I wanted to check and see if anyone else has experienced these as having a low quest-starter drop rate, or if perhaps I'm victim to the RNG: Doomfire: obsidian rock lords, greater obsidian rock lords, Doomfire: elemental shamblers (a cumulus cloud) Vegarlson: earthen emissaries (digested rotting leaves) Vegarlson: gemmed protectors (a diseased worm) Vegarlson: earthen mud elementals (diseased lipids)
I get the cumulus cloud one ALOT, and the obsidian rock lord a few times not a lot, those last 3 from vegarlson though I don't think I've seen any of them at all.
I have only gotten the obsidian rock lords one, a single time. I have gotten the cumulus cloud drop probably 5 times. I have never gotten any of the Vergarlson drops.
I constantly box toons when i'm questing. i've completed all the dropped quests on 4 toons to date (2 sets of 2). The first 2 spent a few hours killing Springview Healers for the branch. The 2nd 2 each got it within 4 kills. Similar rng for other items across the 4 zones. It is completely RNG based. Of the items you listed as having difficulty dropping, none of them stood out for me as being difficult to obtain, on either set of toons.
I haven't found any have a lower drop rate that others. I DO get some more than others.. but that's because I kill some mobs more than others.