I think that it has something to do with psychological development fuelled by religion. When we are young, we intend to think in dualistic terms, good versus bad. We also intend to seek comfort and support form others and a purpose in life.
Add to that a religion in which everybody close to you believes in, which promotes heaven, warns for hell and has an unquestionable authority. Religion seems to offer all the answers to our questions and needs. All other ways of explanation of our world are labelled wrong or better being caused by the devil himself.
So any ideology which offers a goal, a system of beliefs and satisfaction for our psychological needs has a good chance of finding enough supporters, no matter what the insanity level is. example from other religion : Jim Jones and his followers
Islam does it's job well. Many activities are in group which not only lead to strong feelings of belonging together and enough support to handle the daily struggles, but also to enormous social control. In return it only demands to sacrifice several liberties (and occasionally some body parts) through guilt and fear. Among them the most important one of them all: freedom of thought.
The evidence of the disadvantages can how ever be found in the psychological health of the followers. Islamic answers aren't generally deeply meaningfully about how to deal with the inner self, emotions and feelings. You have to suppress them for the sake of Allah ! To point this out, a not daily example:
I remember a discussion I had once with a (liberal) mother who has lost her son years ago. I tried to convince her that he was now in a better place. (I was young and religious, what did I know). She said that she only wanted to see him around in the house. To talk to him, to be there for him.
I think the damage is there, but the person has to allow him or herself to feel it, and not hide it behind the farces of religion. In many cases people have to choose between own grief and the happinesses ( being in Heaven) of the deceased. On the other hand they have to choose between them self and the will of God. How can they be selfish ? So they suffer. . . but it wasn't our choice to be created, never mind the question of why the person had to die in the first place.
An interesting article about burring emotions : google > Why Talk Therapies Cannot Heal
Talk therapies do as much as religion. Don't focus on the pain and don't let it out !
( For the record: I found it a interesting article because it supports my personal view which is based on philosophical/psychological books and idea's. I don't know much about the centre self, nor do I want to. I am not promoting anything. )
The problems with religious belief start when you start to realise the complexity of life and the insufficiency of religious answers
This post expresses my own opinion, I could be wrong.