Why do we automatically conclude that based upon NH results? The players, conditions, styles of play, etc. are all very different to those in today's SH S15. If we look at the steeped-in-NH-conditions-for-long-periods Aus coaches we have recently brought back to Australia, e.g. Williams, Graham, Gaffney, the omens are not exactly highly positive are they?
The 3 guys you mentioned have very different NH coaching records to Cheika.
Williams was still serving his apprenticeship at Munster when he got the call to join Deans in the Aus national team set up. Personally I think it was too soon for him and he would have been better served staying put and continuing to learn. It's one thing to be a forwards coach of a dominant pack as Munster were, but he would have gotten so much more experience with them now when they are no longer dominant and are rebuilding. But I understand why he said yes to the Aus job as when your country comes calling you're almost always going to say yes as it might be the one and only time you are asked.
Graham didn't have a very impressive NH record with Bath and Saracens. Twice at Bath he was overlooked for the head coaches gig and only got it at Sarries because Eddie Jones left a year early. In the year that Graham was head coach Sarries finished 9th in a 12 team competition.
Gaffney has been in decline for years and it was obvious to anyone in the NH that he had lost his mojo. Whoever decided he would be good at the Tahs either never did any research on his performance in recent years or was a buddy of his from the old days.
There's no gaurantee that Cheika will walk into a S15 side and wave a magic wand and make them successful. But he has a much better recent NH record than any of those 3 guys. As LG mentioned his biggest mistake at Stade was not bringing in his own people sooner. I don't think he was ready for the level of politics in French club rugby and should have got his own people in key positions to support him asap. Still he improved Stade's fortunes and probably would have built on that next year if given the chance.
His teams played a better brand of rugby than has been served up by any of the Aus teams in this years S15. But as mentioned there are no gaurantees, but this is true of any coach taking over a new team even if they were already working in S15. It's a new set of players, coaches and backroom staff to deal with.
Do I think Cheika will be an automatic success in S15, no. Do I think he would be worth a punt for one of the Aus S15 sides, hell yes.