A critic may reject some miracle stories as legendary, and not others, with no inconsistency at all for the simple reason that even if one holds miracles to be possible, one need not hold legends to be impossible! There are other factors, literary and historiographical ones, that might lead a critic to conclude that even though miracles can happen, it does not appear that in this or that case they did.

Does it take a blanket presupposition for a historian to discount some miracle stories as legendary? No, because, as even Bultmann recognized, there is no problem accepting reports even of extraordinary things that we can still verify as occurring today, like faith healings and exorcisms. However you may wish to account for them, you can go to certain meetings and see scenes somewhat resembling those in the gospels. So it is by no means a matter of rejecting all miracle stories on principle. Biblical critics are not like the Committee for Scientific Investigation of Claims of the Paranormal.

Fundamentalism fills you with answers before you even think to ask the questions.

If, when we compare two versions of a story, the second known to be a retelling of the first, and find that the second has more of a miraculous element, we may reasonably conclude we have legendary (or midrashic or whatever) embellishment. The tale has grown in the telling. This sort of comparison is common in extrabiblical research and no one holds that it cannot properly indicate legend formation there.

What suggests to non-Evangelical scholars that the resurrection narratives contain legendary accounts? First there is a variety of apparent contradictions in the stories which in any ancient narrative would have to arouse the historian's suspicion.

It is quite likely ... that the central figure of the gospels is not based on any historical individual. Put simply, not only is the theological "Christ of faith" a synthetic construct of theologians, a symbolic "Uncle Sam" figure, but if you could travel ... back to First-Century Nazareth, you would not find a Jesus living there.

The very admission of the need to harmonize is an admission that the burden of proof is on the narratives, not on those who doubt them. What harmonizing shows is that despite appearances, the texts still might be true.