Ice arrow doesn't lower resistance to resistance debuffs, it lowers the (de)buffs the enemy does. So this means that any -to-hit they do will be weaker, alternately, it also affects any +to-hit they have. You can see this in CoD as -Special as apposed to Acid Arrow which is -Res(Special)
Similarly, EMP Arrow does not do -Res(Special). It lists it as -Boost... which I have no idea what that means, but looking at the child effect, it seems to be the same as the -Special that Ice Arrow does (just greater magnitude). I loaded up a power analyzer and jumped into game to confirm this.
Acid Arrow shows up as the Resistances being debuffed, EMP Arrow does not show these.