The stiffness of any wheel depends primarily on the stiffness of the rim and stiffness and number of spokes. For a stiffer wheel, you need either a stiffer rim, or stiffer spokes, or both. For spokes that means either steel rather than titanium, thicker gauge rather than thinner, and more rather than less number of spokes. Unfortunately, all of these choices have tradeoffs: mostly increases in weight, and in the case of thicker or more spokes, poorer aerodynamics. The trick remains to find the optimum balance.
Fortunately, most bikes are not very sensitive to wheel stiffness. Rarely is it a problem for good performance. About the only disqualifying condition for flexible wheels is if it bothers a particular rider. In some cases flexible wheels can be the “last straw” on bikes that are already built with flexible parts (fork, stem, crank, bb, etc.). Additionally, as it relates to handling, uniform flex characteristics are more important than overall stiffness. Non-linear flex can be at best, quite disconcerting, and at worst, overtly dangerous.
At any rate, yes, Easton wheels are stiff enough; in fact they are stiffer than many other wheels, including some rigid spoke composites. Easton’s design has uniform flex characteristics as well.