The book is by William MacAskill, founder of 80000 Hours and Giving What We Can. Excerpt:
Effective altruism takes up the spirit of Singer’s argument but shields us from the full blast of its conclusion; moral indictment is transformed into an empowering investment opportunity...
Either effective altruism, like utilitarianism, demands that we do the most good possible, or it asks merely that we try to make things better. The first thought is genuinely radical, requiring us to overhaul our daily lives in ways unimaginable to most...The second thought – that we try to make things better – is shared by every plausible moral system and every decent person. If effective altruism is simply in the business of getting us to be more effective when we try to help others, then it’s hard to object to it. But in that case it’s also hard to see what it’s offering in the way of fresh moral insight, still less how it could be the last social movement we’ll ever need.
A disagreement could resolve into one side being mostly right and another mostly wrong, so actual harm+benefit isn't necessary, only expected harm+benefit. All else equal, harm+benefit is worse than pure benefit, but usually there are other relevant distinctions, so that the effect of a harm+benefit cause could overwhelm available pure benefit causes.
This is true in principle, but since I take disagreements pretty seriously I think it is normally false in practice. In other words there is actual harm and actual benefit in almost every real case.
Of course the last part of your comment is still true, namely that a mixed cause could still be better than a pure benefit case. However, this will not be true on average, and especially if I am always acting on my own opinion, since I will not always be right.