Well said. I happen to concur with you, good observation that Nadal has a tendency to go far behind the baseline when he does not feel comfortable during the match. The only exception would be clay, where he seems to always play far behind the baseline nowadays and still can display a good level. Maybe next year Rafa will get used to return within the baseline even on clay? After Wimbledon 2019, I think Carlos Moyá and his technical stuff are encouraging Rafa to return within the baseline. It will be interesting to see if Rafa adopts this methodological tactic even on clay.
Correct me if I am wrong, but as far I am concerned Novak is more regular in his return tactical pattern. Novak returns within the baseline in every match, regardless of the court particularities. Regularity in the return execution is a sign of secutirity, which is unsurprising given the fact that Nole is widely considered, and rightfully so, to be the greatest returner of tennis.
I don't feel that Nadal will go near the baseline while returning serve on clay. It probably wouldn't make sense for him. Because of the bounce and speed on clay - Nadal can still hit the ball from far back and can land it deep back in court. His ROS on clay is actually fairly aggressive. He has enough power to hit deep and put his opponent on defensive even while returning from behind. If he stood further up, he would probably be not as effective. The clay supports his spin.
On hards and grass (especially) - it is harder to return from far back. Simply because the ball is travelling much faster and the bounce is lower. Rafa cannot set up his shots many times (esp. forehand) and therefore ends up missing, mistiming or shanking. Even when he connects, unless he has perfect timing, the return lands short in the court, thereby putting his opponent on the offensive.
Novak is a great returner. So was Murray. And even Fed is great. These guys return conventionally. They can use the same grip on both sides as well. They can take the ball on the rise and hit through it. They don't need a lot of prep on their shots (unlike Nadal who has a huge backswing on the forehand). So Novak can be aggressive on ROS. Couple that with amazing anticipation, unnerving accuracy and excellent backhand - that makes Novak as one of the best serve returners of all time.
It is not as much about security. That's how you return serve. Djokovic just happens to be very good at it, because of the above attributes. Nadal is different in the way he returns serve from way back. They don't teach you that in tennis schools. It's something Nadal does which is truly unique -as the rest of his game. Regardless of how much he wins, Nadal will still go down as one of the most revolutionary players the game has ever seen. His technique is remarkable and no one will likely ever play tennis as he does.