“Piercing monsters is also incredibly good with weapon “procs” (such as the Chance to cast Amplify Damage on Witchwild String), as each monster pierced gives a separate opportunity to trigger the effect.”
If using Multishot, the center two arrows of the volley are the only ones that can pierce or trigger effects. The rest of the arrows still leech and count with critical strike.
Strafe on the other hand, every arrow shot can pierce and trigger effects.