Part of the purpose of the Reforged Mythos is to give you the benefits of a particular material, and I've attempted to balance the extra benefits of each Mythos around the idea that some materials' base effects are just better than others - Gold gives you bigger damage dice, Mithral makes you weigh slightly less, for example.
One of the reasons I've been hesitant to finish off/add more material Mythos is because I'm deliberating on whether I should leave them as they are, where we just sort of gloss over where the Alchemical Gold for your Syntrofos that took The Glorious King of Metals came from, or whether I should actually add a little power to the Mythos and require that an adequate amount of the requisite material be supplied by the party during the Reforging. Which could be a big deal early on if you have to track down some elven royalty and barter a small mansion worth of Mithral from them. Then again, it would probably just incentivize starting the game with a material Mythos even more heavily, to the point where it's dumb not to. Which I'm sort of okay with.
Anything is good. I think the Syntrofos needs a lot more Mythos before higher level play can really be tested, though.