If you can't quantify an line of action easily then it's out as a candidate for effective altruism

If effective altruism only allowed working on things that were easily quantified then GiveWell, 80000 Hours, and GivingWhatWeCan would all be unfunded and unstaffed. Most of the benefit of those organizations is unclear and very hard to measure, but there are rough reasons to think that they're very important. Effective altruism is about doing as much good as possible with the resources you have, all things considered. Lines of action that are hard to evaluate do need to be more promising than similar more easily quantified approaches to be worth working on, but that's a pragmatic response to uncertainty.