The simple fact that a guide and client fell 400' tells us just how " safe" this technique really is.
Any technique, used in-approprietly, is not safe. The correct choice to solo, simu-solo, short-rope, running belay, 5th class belay, fix ropes (aid) etc. is not always clear, and is one of the biggest challenges in alpine traveling. Moving slow and 5th classing (leader climbs, places pro, anchors, second climbs) a long easy route could be just as dangerous.
Good Judgement Comes From Experience
Experience Comes From Bad Judgement
I love that quote!