the key difference is the dynamic tackle-zone / breakdown, and, I suspect given how many times our Sam got pinged for it at Bath, to a lesser extent, the refereeing of the technical aspects of a tackle in Union.
The former is not something a professional League player is going to pick up in few weeks. No matter how good a League player they are. And he is very, very good.
My sense is he switched codes in the run-up to RWC 2015 on the promise, a fairly certain promise, of a place in the England setup and side. Perhaps with an assurance that the side, or its backline, would be built around him. Now that is strange given his Union club wouldn't play him in their 1st XV in the position he played at for England. And we aren't talking a 12 playing at 13 or an 11 at 14. We are talking a flanker playing at centre.
Who made that promise, and who allowed that promise to be made, we won't likely know until they've all retired and the autobiographies get written. But what I would like to see, and this is no reflection on Sam, is the summary sacking of every single person in the RFU who facilitated, encouraged, and supported his fast-tracking into the England side. Starting with the coaches, and going all the way to the top. Because anyone with a Level 2 coaching certificate would know the odds against it working out would be very, very long.