In an ideal world I would agree with you (although in an ideal world there would be no paedophiles in the first place) but the Police are undermanned and the fact that evidence from paedophile hunters have been used in numerous cases suggests that, in those cases, they are getting it right.
Agree but with the caveat from Edna below. If they are doing it and handing over evidence to the Police to get the scum off the streets then all well and good but as someone else alluded to, some do it and post the stuff online before any prosecution has a chance.