Nicholson benches are sometimes criticized as lacking mass and rigidity because the top is relatively thin. One of Christopher Schwarz's rule is to always add mass. My bench will contain roughly 111 board feet of lumber and will be 8 feet long by 2 feet wide. Assuming a Roubo with the same surface area, a 5 inch thick top, the same size legs and 2 inch by 6 inch stretchers around the bottom, it would contain roughly 122 board feet of material. To make things almost equal, assume I put a transverse bearer every foot across the Nicholson bench, bringing the total up to 117 board feet.
These benches have roughly the same mass and the same work area. Which is more rigid? Not being a structural engineer, I don't know the answer, though I want to explore the question at some point if I can. Intuitively, there is actually more structure in the top of the Nicholson and the side boards take advantage of the properties of wood to resist stretching and compression along the grain. Torsion boxes are known for their rigidity. I can tell you for sure that a 2 inch thick fir board supported on 1 foot centers is pretty solid. Of course, so is a 5 inch thick plank supported on 6 foot centers. Any engineers care to comment?
Until proven otherwise, I think both of these benches have more than adequate rigidity. That's probably why both traditions came about. Different ways of skinning the cat. My point here is that there is no reason that a Nicholson bench need have less mass or less rigidity than the alternatives. That's a design choice.