I remember being taught that in my IDC and thinking it sounded like a disaster waiting to happen.
While I'm not advocating for the "OW cert in a weekend", I do think it works fine for some students. If students are, prior to their OW class, already comfortable in the water, can learn and complete the skills with few or no problems, there is no reason that the confined and open water requirements cannot be completed in two days.
When I've seen the class done this way (never taught this way myself...) an instructor would typically have just two students, which also helps quite a bit with getting the necessary work done. I've also seen that if everything runs smooth, the students get their certification; if there are any hiccups, they're given a raincheck and have to come back another weekend to finish the work. So the "OW cert in a weekend" is not a guarantee; students still have to meet standards.
And as we all know, the quality of the instructor is arguably the most important factor in the competence of the new diver. A student spending a weekend with a good instructor will be a much better new diver than one that spends a month with a not-so-good instructor.