One argument against the effectiveness from mega charities who does a bunch of different, unrelated interventions is that from the Central Limit Theorem (https://en.m.wikipedia.org/wiki/Central_limit_theorem) the average effectiveness of a large sample of interventions is apriori more likely to be close to the population mean effectiveness—that is the mean effectiveness of all relevant interventions. In other words, it’s hard to be one of the very best if you are doing lots of different stuff. Even if some of the interventions you do are really effective, your average effectiveness will be dragged down by the other interventions.
One argument against the effectiveness from mega charities who does a bunch of different, unrelated interventions is that from the Central Limit Theorem (https://en.m.wikipedia.org/wiki/Central_limit_theorem) the average effectiveness of a large sample of interventions is apriori more likely to be close to the population mean effectiveness—that is the mean effectiveness of all relevant interventions. In other words, it’s hard to be one of the very best if you are doing lots of different stuff. Even if some of the interventions you do are really effective, your average effectiveness will be dragged down by the other interventions.