Just to correct this for the console players, on PC, Mythic troops appear in Glory, Gem and VIP chests. They have zero drop rate from Event chests. The first week of a new Mythic means that any Mythic drop will be the new Mythic. After the first week, a Mythic drop can be any of the released Mythics.
In a nutshell, if you’re in the mid-late game and got all the non-Mythic troops you want ascended to Mythic already, you might want to consider saving Glory Keys and Gems for Mythic releases as if you’re unlucky, it might take thousands of keys before getting a new one to drop in that first week.
To use Gems to buy gold is as big a waste as maps and souls, your gems have one grand purpose, GEM CHESTS and EVENT CHESTS (and for those that have spent $$ VIP chests). It costs 38k to buy 10 gems in tasks so for the big pack of gold (30k) it takes 140 gems which cost you 532k. Your 50 gem chests, or event chests will give you better troops and is why most of us in great guild have most cards mythic before lvl 300
You’ll get to a point where you don’t really need Gems for chests and accumulate a lot of them. Is it cost-effective? No, but it is the best choice with a redundant currency.
I’m an end game player
Level 509, in a top guild.
166 troops mythic, 5 5*, 14 3* cities.
I pretty much ignore tasks usually, no arena or maps. Just pvp and I donate all gold to guild. Currently saving gems, keys, and glory for new troops. Donate around 800k gold a week to guild.