Lots of these rigid vangs break, compared to rope vangs
which are more reliable, obviously. As OZ stated booms often
break right at the vang attachment point. This is a weak point
because of the leverage, as CM stated.

Not if installed correctly. I had my Quick Vang boom bracket break loose from
the boom once as we crossed the starting line of a race. The reason for the
failure ..... the nitwit who installed the boom fitting attached it to the thin
walled boom with machine screws and no backing plate. I would be all of 2
threads had been holding that fitting on. I drilled and tapped proper sized
holes in a 5' long piece of 1/2" x 1/2" aluminum bar stock and placed it inside
the boom. Now those machine screws have much more meat to get a grip on and
the 5' length really spreads the load. We've done a couple of auto jibes in
big winds on downwind legs with nothing breaking since then.
S/V Express 30 "Ringmaster"
"Trains are a winter sport"