I refer to the basic beliefs about Marcion, Valentinus and other Gnostic beliefs.
When the various Justin, Irenaeus, Epihanius etc wrote, they were doing so already from a position of relative force, when their enemies had already been confined. Force generates security and security generates a more serene objectivity on the positions of the enemy.
An example may be the representation of Hannibal by the Greek-Roman writers: the greek Polybius was surely more objective (and less propagandist) than the Roman Livy. Polybius came before Livy. But even if Livy describes Hannibal as a very bad person from a moral pov, even so he recognized his greatness in the military field.
So if the analogy is correct, even if Marcion was described as a negative person, at least the basic facts about his original belief had to be reported more accurately by the his proto-Catholic opponents. And even more so from the writers who were more distant temporally from him.
Why the reports about the heretical beliefs are basically accurate
Why the reports about the heretical beliefs are basically accurate
Nihil enim in speciem fallacius est quam prava religio. -Liv. xxxix. 16.