There are indeed levels of criticism of Christianity. I've written on the Christ myth theory (links below), and I have a series coming up that explores it further, including a long article comparing Jesus to Osiris.
So the overall criticism takes the form of a dilemma: either you take much of the New Testament for granted, for the sake of argument, in which case most Christians turn out to be grossly hypocritical, or you doubt Jesus's historicity altogether, in which case Christianity is no longer so special a religion.
I level both criticisms. The scantiness or ambiguity of ancient historical evidence means that we should should reduce our level of certainty, I think. Thus, the different kinds of criticism can be pulled out to suit the occasion, rather like how a lawyer will use different, even contradictory arguments, depending on the vicissitudes of the trial.
The particular Christ myth theory you're talking about (the Roman conspiracy) I haven't read in much depth, but I'm aware there are a few books on it. I suspect that conspiracy would have been redundant after the fall of Jerusalem since Jewish sympathizers would have been disposed then to pander to Greco-Roman culture in any case.
