Not actually used any alchemy yet, the diagram was my attempt to see want i needed to do.
From brief looks with dfreveal i have seen lots of frost crystals and a few flare crystals but very few of any other type (though if may just have been my maps) and a few of the other minerals seem to be both rare and found in very small clusters. I've also seen almost no rock salt on the couple of maps i have played properly, which prevents all the special weapon materials, a few extra reactions to produce intermediate reagents from trace elements in common stones may make alchemy a bit easy to use in practice (even if trace reaction only produce 5% of output so need 20 boulders & possibly 20 fuel)
Part of the imbalance i think is because the amount of special metals/crystal weapons and armour i have collected from ambushers in the second year of my fort if more than i could get from completely mining out multiple levels of the map.
As a final not i like the couple of evil races coming to trade, after all our loyal dwarfs (i.e. the ones not swimming in magma!) don't care who if bringing them stuff. And the get a ringside seat when foxes, hobgolbins? and elfs send caravans at the same time; the elves avoid the depot safely until an ambush happens, which is chased by the evil guard brings them too close to the elven caravan, which gets accidental slaughtered by the evil guards. Diplomatic chaos thus endues.