The example they show of a Great Table is, to my taste, way too busy. Here is my unsolicited opinion:
The top and bottom horizontal rules on the Title appear to be superfluous, and I dislike how it is aligned with the first column (row labels) rather than the second. I feel like a little space to breath at the bottom, along with a bold font would add visual hierarchy w/o the clutter.
The row label backgrounds are far too dark and the font weight makes it hard to read. I'd prefer a very light blue here instead. I don't like the row group label ("Name") being italicized.
The spanner labels floating in the centre make the table hard to scan. Would be much nicer aligned left.
Finally, I really dislike the font (maybe this is just my browser, though).
I mocked-up some of the changes here, I think this is a much easier to read table:
https://i.imgur.com/iMMf5vo.png