Although I don't have any official figures to hand, the failure rates are very low and the situation is very different to KAL, when they had the problems with malicious 777 trainers. Training itself is pretty much non-existent; you'll be expected to have studied and know the answers without any instruction from your trainer.....it seems this is the 'way' in the East. When you are line training you'll fly with 'Western' trainers for some of your sectors and this is your opportunity to ask all the questions you were never able to ask and try to figure out what the Japanese were talking about when they asked you awkward questions that you were unable to answer.
Now, although actual training doesn't exist in the way that you might be used to, the Japanese instructors are, for the most part, honest, friendly and not vindictive. There are no politics involved in trying to oust the 'Western' pilots as was the case with the KAL 777 fleet. So, all in all, a completely different scenario and one that doesn't bear comparison.
Back-to-backing is still possible. It is used regularly by some pilots and occasionally by others. I'm more of an occasional user (maybe twice per year) but one of my mates does it almost year-round. I've never had any problems with doing B2B days off and I've never heard of anyone else having any snags. It's good because you get a decent time off at home AND you get to keep $2000 for one of the month's commuting allowance but, on the other hand, you've got to 'pay' for that time off at some point and 5-6 weeks in Japan can be a killer
