I am the liaison between my hospital and several different schools
who use my hospital.
The "best" school in our area was using 12-hour shifts, but stopped for several reasons. One of the reasons was that the students did not seem to be learning as much. With only having 1 assignment for the whole 12 hours, they weren't getting enough variety of experience even though the total number of hours in clinical was the same as 2 6-hour sessions.
The students who had 2 days per week of 6-hour clinicals got more exposure to a wider variety of patients and procedures. If they had the same patient both days, they saw how that patient progressed from one day to the other. If they didn't get the same patient assignment, they got to see an additional patient. Also, the 12-hour once-per-week clinical schedule folks seemed to lose more of what they learned in between sessions.
We have seen a similar phenomenon in staff development in the field of orientation. We have found that orientees working only 3, 12-hour shifts per week tend to progress a little more slowly and seem to regress a little more when they miss a few days a few days due to illness, vacation, etc. The 5, 8-hour shifts per week people get a more frequent reinforcement of their learning and simply get more repitition of meeting a new patient, doing an initial assessment, etc.
You simply get less experience with 12-hour shifts --- and the experiences are farther apart and therefore reinforced less frequently.
I'm not saying that it can't be done well ... but anyone considering it should be aware of these issues and take them into consideration in the planning.