I have made a few strange observations but when there is a resource that I do not need, I get plenty of it whilst farming. Those that I need I don't seem to get so I made a small experiment I sold and used my 2* blue evo shards to the point where I need 1 to evolve one of my character.
I played about 15 LOL Blue levels and farmed all day long not a single one appeared... I used to get these all the time...
Then I looked at the few lucky ones with Viking Eddie as Trooper Champion, a lot of us are spending ironite like crazy to get him but those who have him have him 2 or 3 times... There is randomness, there is luck... but there might be something else....
Anyway, I know wants corrupt ox cultist, harpies, undead allied soldiers and dogs