If I were to argue that we cannot be certain about Matthew's account of the guards at the tomb both because none of the other gospels corroborate it and because there is an obvious, plausible motive for its invention (i.e., to counter claims that Jesus' body was stolen), no liberal scholar would bat an eye. Moreover, even many conservative scholars would see the need to offer affirmative reasons for believing Matthew's account.
On the other hand, if I suggest that we cannot be certain about Galatians 1:19's reference to James as the "brother of the Lord" due to lack of early corroboration (i.e., no other 1st century writing describes Jesus' brother James as a leader in the movement) as well as an obvious, plausible motive for interpolation (i.e., to clarify which James it was that Paul met), I am met with howls of outrage from liberals and conservatives alike. Moreover, my argument is deemed to be so spurious that it does not even warrant a response.
While I am happy to concede that different factors may lead to different conclusions in each case, the basic logic and structure of the two arguments is nonetheless the same. I am hard pressed to see why the former should be uncontroversial while the latter is viewed as some sort of dirty trick.