Id say it would be the scale required (20nm)to be an on chip...

Id say it would be the scale required (20nm)to be an on chip solution to replace the sram and Nvm in 1( nvm currently sits off chip i believe due to scaling limits.) there is an excellent diagram on the cousins website which illustrates this perfectly. its from a media article in regards to scaling edge Ai with Reram and is pretty much what 4ds is aiming for ,
search electronic specifier if you wish or go to the media coverage section at WBT its worth it.
Im sure someone more technically may be able to expand further .