My thoughts:
I liked the idea of Geostrata. But as I looked around, the deposits looked like vanilla ore blobs, scattered more-or-less randomly, with no real sense of reason visible.
In comparison, underground biomes behaves very differently -- using the new version to make a "large underground biome", the behavior of UB to give primarily one stone in a very large area, with good-sized layers of additional stuff, is very clear, and stands out significantly from the "scattered, random blobs" that I see in Geostrata -- enough that I'll probably use UB instead of GS. (EDIT: asking my players on the 164 test servers, only one other person expressed an opinion, and they agreed with me).
But the colored trees? The rainbow biomes?
I think the colored trees are too frequent; I don't want rainbow biomes in the overworld, but I do want them in mystcraft. I may very well use the "only spawn in biomes, disable the biome, find it in ages" technique.
But the crystals? The magic?
When you get to the heart, I'm more likely to want the trees, and NOT the stone nor the crystals, and I don't see any real use for the magic (we have no Thaumcraft taint to counter). But if the rainbow biome is in ages, then the instability fighting feature figures to be fun.
So, perhaps a better question: Instead of splitting the magic stuff into dyetrees, why not split the magic stuff into Geostrata?
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I don't think that the crystals are inherently OP. I think that the difficulty of finding them in the nether is perfect, and that we determined that they will generate too much in an ATG overworld (gravel beach primarily). I can strip them from the overworld (COG), just not sure that I want them in my server at all.
(Currently reading the whole thread, just read page 1.)