Something‘s off with the ratio acquiring duriel mats. Did 1h of helltide farming, opened 12 chest (for the season journey achievement) and got 20 steel out of it. That results in 5 Grigoire kills aka 5 duriel stones, but effectively only two duriel kills.
If i invest the same amount of time into tree of whisper quests, world bosses etc, which is part of your normal gameplay loop anyways, you end up with at least 20 duriel eggs, which nets you 10 duriel runs(in theory). So imho the balance is way off concering the ratio of farming duriel mats. varshan mats come naturally during your gameplay, whereas cinders aka steel is much more tedious and boring to aquire.
So either rethink the whole system or
lower the summoning ratio for grigoire or
add more monsters to helltide or
fix the cinders given per monster kill.
bosses in helltide zones should give way more cinders. btw the little events are also inefficient considering time investes vs cinders gained.
Apparently the uber unique drop rate was buffed in the overworld/dungeons, but after 500hrs of playtime still haven’t gotten one.
Which in conclusion makes me wonder how the hell did all the top players in the gauntlet leaderboards aquire all those shakos and starless skies 
Answer they play 24/7.