Yeah, it's hard to say with any certainty. I agree that the far end is the curve probably looks nothing like the "neighborhood" a couple percent away, relatively speaking.

I also agree that the variety of data plays a big part in its complexity. If you have a few petabytes of data, but it's really only a handful of tables you can real hone in on the relationships. If it's a wide array of sources with many tables between them then you have some nasty problems like entity resolution.

All happy data sets are alike; each unhappy data set is unhappy in its own way.