The problem is in the scale of size of PADI - they are so large that it is very difficult to keep a tight quality control system in place.
GUE has excellent quality control because they are small enough to make refresher renewals part of the SOP. As a result bad instructors weed out or are corrected far quicker.
But the science, development and materials PADI puts out now are tremendous.
GUE requires every student to complete their QA form before issuing a card. On it are various course components important to the agency management. Did the instructor do X? did he avoid Y? Instructors violating standards are caught fast, like within one or 2 bad classes.
No other agency at any level does this kind of course-by-course review of their instructors through any kind of student follow-up (at all). None of my PADI, SSI, TDI or UTD classes had any such QA review at all. I have not personally taken an IANTD, NSS-CDS or NACD course. Some of these other agencies are comparable in size to GUE; they are just uninterested or unmotivated to be rigorous, their purpose in many cases is to support instructor's businesses and issue cards. Rocking the boat, remedial teaching lessons for poor instructors, or kicking instructors out isn't part of their business model.