Most sadhus are authentic. Of course there are fake ones, only the probability of the fake ones being Nagas is remote given that there is nothing to gain from looking and behaving like a Naga. They are not the most popular people around.
Indian poverty did create a lot of fraudsters. There is Swami Agnivesh a prominent fraudster, TV material. There is also the Shankracharya of the Dwarakapeet who I believe is a fraudster. So no argument that there are no frauds, but just that we need to be a bit more discerning instead of brading them all the same.