I think the UB thing is at least arguable in some cases, certainly. But I think the ISA extension thing is absolutely a good idea for the simple reason that if

I think the UB thing is at least arguable in some cases, certainly. But I think the ISA extension thing is absolutely a good idea for the simple reason that if that ever changed you would be -screwed- and forcing people to Think Really Hard about their usage if ISA extensions is probably a net win anyway. Possibly they should be in a second "bucket of scary stuff" with a different set of hazard stickers applied, but using unsafe for all of it makes sense from the POV of 'unsafe means I am voiding my warranty inside this block' rather than anything more specific.