There was an interesting talk I heard a while ago about the scalability of oral exams vs written. They did quite a bit of study of typical exam preparation and marking times and the cross over point where oral exams were actually taking longer than written exams was surprisingly high, around 180 students or so. To be fair though I think they considered 15 min oral exams, but the gist is that in the scalability argument people tend to forget to account for marking and exam design time appropriately.