This is a confusing question for me. The same factors relate to food forests as with any other landuse. There’s the possibility of introducing species that might be problematic (invasive, poisonous, and so on); people can fail to follow the patterns well, by planting so closely that there is not enough light and space for flowering and fruiting, and so minimum productivity in terms of harvest; or fail to create enough access ways for harvesting and maintenance, so that the forest is only a forest, without the food product!