The real difficulty is that we only have in our possession what amount to being third century reworkings of original source material that is now lost. Think, for example, of Justin's Syntagma. Indeed, what's worse is the fact that few scholars think through the implications of their own research. For instance, most would agree that Book One of Irenaeus's Against Heresies has reworked a large amount of material from Justin's lost work in the section which follows the account of the Marcosians. It is difficult to tell what was part of Justin's original work but the general notion of such a borrowing is basically acknowledged by everyone.
Clearly then Church Fathers were pointing to the existence of sects which they had little real first hand familiarity with. Take the various reports about the 'Phrygian sect' (= the Montanists). How many Church Fathers who make reference to the Phrygians actually visited Phrygia to see whether or not there was a large number of sectarians there? Almost none I bet. The same thing is true with respect to the Marcionites and Tertullian's 'definitive' five volume report on the sect. He never even so much as once came face to face with a 'Marcionite.' He's just reusing sources, accepting the authenticity of material he could not possibly 'fact check' in any way.
Now let's turn to Clement of Alexandria. He mentions 'those of Marcion' and Carpocratians too. Yet Origen interesting admits that he had been all over Egypt and never ran into a 'Carpocratian.' What gives? Clement was just recycling lost source material which provided a general classification of the different types of 'heresies' that were out there without any real first hand knowledge of the sects in question. Yet what were these sources that Clement used?
There seems to be no evidence that Clement actually read Irenaeus's Against Heresies. Nevertheless he seems to have come into contact with a 'heresological work' which provided the background information for most of this which appears at the end of the Stromata usually dated to the beginning of the third century:
Of the heresies, some receive their appellation from a name, as that which is called after Valentinus, and that after Marcion, and that after Basilides, although they boast of adducing the opinion of Matthew; for as the teaching, so also the tradition of the apostles was one. Some take their designation from a place, as the Peratici; some from a nation, as the of the Phrygians; some from an action, as that of the Encratites; and some from peculiar dogmas, as that of the Docetae, and that of the Harmatites; and some from suppositions, and from individuals they have honoured, as those called Cainists, and the Ophians; and some from nefarious practices and enormities, as those of the Simonians called Entychites. [Strom 7.17]
There is absolutely no way that Clement could have been reporting the existence of a great number of sectarian groups that just so happen to put forward the very same reported in other Church Fathers. This has to come from some sort of ancient compendium, a precursor of the Philsophumena or perhaps its original source. The most likely source - Justin's Syntagma.
BTW I am fairly certain that the 'Peratici' are from Perea despite the wild theories of the guy writing the Wikipedia page on the subject. Notice that Clement specifically indicates the group 'takes their name from a place.' Perea and that region to the east of the Jordan river must have been the breeding ground for earliest Christianity. The Gospel of the Hebrews seems to have come from here too.
