The hard gainer training philosophy is a myth! Here's how it works: If someone has great genetics like ronnie coleman or dorian yates they both grow by using either higher volume or lower volume. If someone has poor genetics they grow slow whether they use low volume or high volume and higher volume over a period of time ruins their joints-hence slowing down their progress. Both ronnie coleman and dorian yates were forced to retire due to injuries. Ronnie over did the volume and it eventually caught up with him and dorian trained beyond failure which eventually caught up with him. Both of these men would have maxed out their genetic potential using moderate volume (6-12 intense work sets) and i believe they both would have still been competing if they had followed that approach and yes they would have been just as big and probably even a little bigger due to keeping injuries at bay.
So, the whole hard gainers routine is a myth. Training with high volume will not benefit hard gainers as much as easy gainers but neither will moderate or low volume. It's not their training routines but rather, diet, drugs and genetics that makes them huge (mostly genetics). Coleman and Yates would have been massive doing 8 worksets sets for quads once a week and they both would have been massive doing 12 sets twice a week. Moderation is the key for all genetic types and it saves time in the gym and prevents injuries. BODYBUILDING IS BEST APPROACHED LIKE SPRINTING NOT A MARATHON RACE, REGARDLESS OF ONE'S GENETIC MAKE-UP!