Just going back to Leorio's ability, I remember now that the boxer had a small radius of effectiveness, and also needed the Nen symbol on the canvas to boost his ability. Leorio's seems to have more range, even if somewhat slower and visible.
You know this entire mess may actually be averted if Killua told the family about his plans to get rid of Alluka's power. The assassins are powerful enough to protect themselves anyway without Alluka's powers so it's not like it'd be such a big loss to them.