GiveWell’s research into long term effects is noticeably much less than their research into short term effects—even though the long term effects dominate in the long run.
You might be interested in this transcript of a discussion between Holden Karnofsky of GiveWell, several people from Giving What We Can, and a few other EAs: http://www.jefftk.com/p/flow-through-effects-conversation
You might be interested in this transcript of a discussion between Holden Karnofsky of GiveWell, several people from Giving What We Can, and a few other EAs: http://www.jefftk.com/p/flow-through-effects-conversation