The only thing I can think of to cause that many faulty welds would be the use of the wrong rods or filler material.That wouldn’t show up on xrays or even the inspector looking at it. That wouldn’t be the welders fault or it is possible the filler material was substandard, not to specs and possibly mislabeled by manufacturer, maybe even on purpose so they say knowingly but they do not mention who knew.
Could be, but the welder is ultimately responsible for the weld. Every weld had a rod slip and welding rod was drawn from the rod shack and signed off by the person issuing the rod. Then it was checked by the QC during the inspection process.
It was not uncommon to get TIG wire that had some oxidation on it, but that was easily cleaned off prior to welding. When te weld was completed, all the rod used on the weld was turned in and the stubs disposed of at the rod shack. Low hydrogen rod, inconel, monel, and stainless stick rod were always kept in ovens at 200 degrees to prevent moisture. It was checked out in portable rod ovens.
Other than inspectors, substandard or wrong materials are among the overlooked (here) other possibilities than blaming the welders.