I actually think that their main problem is the belief that they can learn everything about the world by reading stuff on the Web. You can't understand everything by reading blogs and books, in the end, some things are best understood when you are on the ground. Unironically, they should go touch the grass.
One example for all. It was claimed that a great rationalist policy is to distribute treated mosquito nets to 3rd-world-ers to help eradicate malaria. On the ground, the same nets were commonly used for fishing and other activities, polluting the environment with insecticides. Unfortunately, rationalists forgot to ask people that live with mosquitos what they would do with such nets.
One example for all. It was claimed that a great rationalist policy is to distribute treated mosquito nets to 3rd-world-ers to help eradicate malaria. On the ground, the same nets were commonly used for fishing and other activities, polluting the environment with insecticides. Unfortunately, rationalists forgot to ask people that live with mosquitos what they would do with such nets.