What feels least sufficient to me are systems for learning Scrum, or XP, or any blend of practices/patterns/principles well enough to do more good than damage.

What feels least sufficient to me are systems for learning Scrum, or XP, or any blend of practices/patterns/principles well enough to do more good than damage. We therefore generalize from poor data. We draw conclusions from poor execution during the learning curve. Another big issue for me, in SW Dev, is the opacity of "code asset" deterioration as we start to iterate features through it quickly. Scrum can exacerbate that. Once we start cranking the Sprint crank, poorly disciplined programmers can hack the code to bits even faster than before, without stakeholders noticing. Hence agile wags once called Scrum "XP without the P".