I don't know how the arrangement was, but my impression is that their activities were frowned upon, not just by those who suffered by them but by Christian morals went against them. And those matters no matter how some wants to tell us that everyone was a hypocrite sinner just like us today. I do believe they at least had some expectation that they collect the amount that they could have collected themselves (which was regulated by laws).
But since people weren't saints either I think nobles and kings could see them as practical tools at best if they had pressing needs for funds. They looked away, and if things allowed they served justice.
Serving justice might not had been so straightforward. Because tax collectors could have powerful friends, other employers who needed their service.
Just like bankers who financed rival feudal lords, kings and emperors, and used them against each other so they can't get rid of bankers.