I respect what they're trying to do with the handball rule, and this is actually a great first step towards it. They'll fuck it up, because it's still FIFA / IFAB, but at least they're focusing on the right problem and more or less looking at it the right way.
The final "unfair advantage" caveat, however, is a loophole that sinks the whole operation. You can drive a truck through that loophole. With that kind of ambiguity, referees and players will still default to the old zero-tolerance habits and claim that this-or-that unintentional contact must be called on the defender.
(A) Consider this not-uncommon case today: a defender closing out on an attacker looking to hit a cross charges at them with arms behind his back. As the cross goes up, he turns his body so he doesn't risk getting hit square in the face; his arms behind him end up getting struck, where they wouldn't have absent him turning. Clearly unintentional - and just as clearly, called handball today. They might argue it made him "unnaturally bigger" (except that he was already trying to make his wingspan unnaturally smaller), but the explicit rule should take that away now. However, they can just claim that the turn let him deflect a ball that, absent his hands, would have gone directly towards a goal-scoring opportunity - so, unfair advantage, call the PK. Takes away the spirit of the rule if you start arguing over whether there was an advantage created and whether it's fair or not.
(B) Really, every single one of those "NOT USUALLY" sub-bullets get kinda swamped by the last one. They were going to ground, it hit their support arm - as a result, the ball was deflected from its target on-goal, which must be an unfair advantage, so call the penalty.
(C) this does nothing about players who chest a ball, misjudge it slightly, and it actually hits towards the side of their chest and might incidentally contact their upper arm or shoulder area while they chest it.
(D) what would this do about the Sissoko handball at the start of the UCL Final? He's gesturing to a teammate, oblivious to the fact that Mane was looking to cross, so it's clearly unintentional. Just as clearly, the arm is at shoulder height in a position that isn't "natural" in the sense of "relaxed" (but it's "natural" in the sense of "people ordinarily make these gestures"). So you've still got a situation where it's in the attacker's best interests to try and fire at a defender's arms to draw a call, instead of trying to score.
Again, I like the general approach, because they're flipping the logic on its head - it used to be that ANY hand/arm contact was a foul UNLESS it met certain exceptions, and they're now trying to say that any unintentional contact is generally going to be OK UNLESS it meets certain specific conditions where it's a foul.