Sandi gets called for blocking because he is usually trying to block the other ruckman. If he takes away their leap, then they have no chance of winning a contest. It's very effective, but the consequence is that sometimes he gets called for it. On balance, it's still worth it.
NicNat's approach to rucking is to run and use his jump to hit the ball at it's highest. It's also very effective, although not against Sandi, because Sandi is bigger and an expert blocker.
NicNat's rucking style is the style the AFL would prefer. Done well, it looks awesome. A couple of huge blokes banging and tangling with each other at ground level just doesn't have the same aesthetic.
NicNat, like a lot of players who can jump, uses his knee in marking contests when he has the sit. One of the great parts of our game is the big speccy. Occasionally there is collateral damage. Sandi wore it on this occasion, but I'm not sure you want to take that out of the game. If NicNat's attempts are unrealistic, he'll get pinged. On this occasion, he actually marked the ball, so I'm not sure on what basis the umpire was supposed to ping him.
Sure, it's annoying when NicNat takes a pack mark, but fortunately, you'll probably only have to endure it once or twice more this whole season.