I had a 1.7 pack with custom COFH controlled world gen, and I had a blast setting it up as well as playing it with some friends. Veins were set to be quite large and very spread out, with the intention of trivializing most large scale quarries without at least a few exploratory digs to locate the ores. That said, I left small (3-4) clumps spread out so people could get started with the basics before they located a vein, which ended up being a life saver. I put most of the world's diamonds inside of obsidian geodes, making sure we had a few ways to break into them without said diamonds.
The pack ended up being a total bust from a pack development and progression point of view, from which I learned a few good lessons, but the ore gen made the pack special and memorable to all (probably moreso to me, since I spent more time configuring and testing the ore gen than we did playing the pack as a whole). At some point, I'll go back to changing the way the world generates for a new pack, but it's not in the cards right now.
I do like the idea of some ores being biome dependant, but then I remember all those times I had to walk 3k just to find a particular biome, and suddenly that doesn't sound so appealing. The idea of surrounding veins in a loose cloud of poor ores seems interesting, but not every ore in a pack will have a poor variation, which is a bit of an immersion breaker. I still ponder the best way to distribute to make it challenging and scarce but still accessable enough for progression. Similar things should be done using Nether Ores as well, since I rarely have a need to go back to the Nether once I've acquired a bunch of quartz and a Blaze spawner, and having profitable mining in an otherwise scarce Overworld would be a good driving force. Still something to think about.
I would enjoy it if FTB was to do the work for me, but I'm fully aware that it's a niche market and a ton of work, which isn't usually a recipe for success. Once I get a decent script for world gen written for a particular version, it's something that can be copy/pasted into whatever pack I want, so it won't matter if FTB, Reddit, or anyone willing to share does the work in the end.