well, there are two basic principles behind 912:
1.working out a lot burns a lot of calories (which is good for getting rid of that annoying bodyfat).
and 2. working out a lot (heavy, frequent stimuli) helps retaining lean mass.
comparing the effects of heavy strenght training and cardio on EPOC, it appears to me that HST-like strength training is more effective for burning calories than any form of low intensity cardio (I'm speaking of LSD, long steady distance, not HIIT or interval training, which might be quite useful on the other hand).
see for example
schuenke et al. Effect of an acute period of resistance exercise on excess post-exercise oxygen consumption: implications for body mass management. Eur J Appl Physiol. 2002 Mar;86(5):411-7.
for further details. this study investigated the effects of a 3 sets, 10 reps to failure training protocol consisting of only 3 exercises (bench press, squat and power clean, 35 minutes in total per training session) on it's effect on resting metabolic rate following 48h after the initial workout. mean increase in metabolism was 20% in these two days. this is a whole lot of energetic demand for regeneration compared to cardio, which has not been shown to have any prolonged effects on metabolism.
all the other stuff in the article is rather supposed to server as an inspiration on how to conceive a training routine that provides the above stated criteria. if you don't like working out only 12 days in a row, go ahead and change the plan the way it fits you best (if you think you can handle such volume, frequency and load for more than 12 days without burning out). yet only one cycle of 912 was never actually supposed to form a stand-alone diet by itself. normally, a decent diet would consist of a sequence of such cycles.
a lot of people got sore joints and tendons from 912, while I have never heard of any serious injuries. I guess it's a blatantly stupid idea to suddenly increase every parameter of one's workout by 300-400% without further preparation. of course you need the general physical preparedness to sustain such poundages every day. this is just common sense.
I can't provide any empirical data in english language, but there is plenty to read on the german forums about 912. generally, people don't enjoy it ("too much, too heavy"

, but no one I ever heard of has had disappointing results in the way of fat loss.
in the end, it's just a question of caloric balance. if you eat 2000kcal and metabolise 3000kcal, you lose 1000kcal equivalent of bodyfat. 912 or any other routine simply helps wasting more energy.