Okay are schemaless databases really such a bad idea? Where does the hate come from? I just implemented an EAV based system and it was and remains terrible, but

Okay are schemaless databases really such a bad idea? Where does the hate come from? I just implemented an EAV based system and it was and remains terrible, but the requirements were to allow users to create arbitrary fields, and to use MySQL. It seems like mongodb/(xyz norel system) would have been such a better choice. I don't know what other options exist. In our case I think it was blind fear that drove the decision not to use mongo, but I see enough people talking them down...