I think this could be a convenient excuse that keeps some from donating more money (I'll wait until I can spend time creating my own charity / doing some hands-on vetting on who gets my money). There are already plenty of organizations (GiveWell, OpenPhil, etc.) that have very good research on the impact different charities have, often with RCTs on those interventions proving how much each $ buys (in terms of quality-adjusted life years or else).