BS”D
I have personally assisted a victim of shadchan malpractice, where the shadchan knew very well that one party to the shidduch was mentally ill and being manipulated by relatives who needed to hide the illness. Yet, this shadrechan went along with it and an agunah situation with blackmail resulted. (The shadchan involved got a nice comeuppance from shamayim too, but that is besides the point).
Paying for successful shidduchim without waiting to see if the marriage works out is bad enough but tolerable. I can just envision what kind of dregs of society will nill themselves as shadchonim if parents were to pay for “attempted” shidduchim.