One concern, as I understand it, with the darker colors of quartz is that there more of a chance for mottling. This mottling is from the plaster, and not the aggregate, but because the smaller quartz is less prominent in the Quartz finishes than the larger pebble aggregates, the mottling can show up more. When properly installed, managed at startup and maintained, mottling is not an issue. The install is the key, in any product, from what I've read.
I may be way off, so please correct me if I'm wrong.