Well the fact is that there really is no accurate predicting when dealing with something like this. They could land in our backyards and tell us our model of physics is competely wrong for space flight and that FTL speeds are not only possible but easy to access. Who knows.
Anyway, I think if benign aliens were to visit us, the main cause of problems would be us. They might come bearing universal knowledge, and of course humans don't generally want exact knowledge, they want their own version of knowledge. In other words the aliens might tell us stuff that we really don't want to hear (ex: 'There is no god or higher being. Take a gander at this document that proves it'), and who would we be to question them? I suppose I don't have to describe what sorts of unfortunate situations that could lead to.