There is some interesting material in here. Wood Handbook--Chapter 7--Fastenings (fs.fed.us). It seems that until 1991, the US standards relied on some empirical tests which gave what seem to be pretty radically different shear results for nails versus screws (if I follow table 7-4 correctly). Then they switched to calculated values, and I can't see any factors in the equations which would lead to dramatically better performance for nails over screws in those (Table 7-5, again if I follow that correctly).
That's all with root dimension being equal, so for fixings like joist hangers and nail plates which fix the max outer diameter, nails will have an advantage which may be another tributary giving rise to this belief.