American Airlines' training program was reviewed numerous times by the manufacturer prior to the accident
It really bothered me to think that that might be prevalent viewpoint - a half baked IOSA audit would serve you better than a manufacturer's visit. The first part of my engineering career was with the manufacturer's. The second part is with the the Airlines. From the airline perspective I was shocked at how mediocre the manufacturers feedback is.
1 week of jumpseat flying on the line and a report/debrief at the end of it is almost worthless. The don't look at why you do what you do. They don't look at your mod status, your FDA data, your crew experience/history, your route structure, your .....etc.. The only value you can expect the visit is havig somebody there from AI or Boeing or Canadair or EMB or ...to answer questions directly.
If they're comments/feedback on your operation make any difference to your operation in the final analysis, then you must really be screwing things up. Manufacturer's visits are usually nothing more than a cloaked marketing ploy.