Design. Look at all of the relief in the very center of the design on both the Mercury obverse and the eagle reverse. If everything is in high relief, it will take a great deal of pressure (or multiple strikes) to get the metal to fill that area properly. With the *regular* Mercury design, they had problems bringing up the bands in the center of the dime. The fasces design on the dime doesn't have anywhere near the relief of the proposed eagle, although the relief on the obverse was certainly respectable. The 1921 Peace dollar had the kinds of problems that I envision here. I know zero about the striking characteristics of palladium.