As others have said they have some security concerns (I don't know enough about that stuff to know how justified or surmountable those concerns are). However with the big manufacturers all locking down their devices more than ever I wonder will they have much of a choice in the end. We're going to need a manufacturer (or preferably several) to actively stand behind the possibility to use custom ROMs, and at the moment Fairphone seem like the only one who might do that.