Transistors per cost and performance didn't scale well to 20nm planar which is why it saw limited adoption, however this apparently improves a
lot with the FinFET nodes, so it's not quite so doom-and-gloom. Yields are always a potential issue with bleeding-edge nodes, and Nvidia always complains, however this time there will be some maturity on both nodes because of the mobile processors having been in volume production for ages now.
Maybe it's because it's not about efficiency of products from their favourite brand?