I wonder if people still reckon Marmin is better than Ben? If so Marmin must be a superstar!
It's easy to see why he' so hard to tackle: Fotwork, agility, speed and second to none positional play. He seems to have a natural ability to find gaps.
In kick returns he's keeps defenders in two minds. He has a decent step and incredibly good acceleration off the mark, so all it takes is a few missed tackles against defence, and with his speed he can put a good amount of distance between himself and other defenders. E.G both long range tries the winger and centre on that side have missed him (so effectively the two fastest opposing players on that side). Forwards and second rowers are unlikely to catch him, so if it's put down in a corner and he gets past the winger and centre, then you know it's time to get off your seat. Even then if others start to catch up to him, Reynolds, Morris and possibly Inu/Perrett are there in support.
In general attack my guess is his size helps him. Defenders are too busy looking at the immediate player instead of where Ben is hiding in back play. Being a brilliant positional player, any offload and small gap due to commited defence, and he's through and off.
Such an exciting player to watch.