https://minecraft.wiki/w/Redstone_Comparator#Fullness_of_containers
Every single inventory block in minecraft emits a fullness signal when hooked up to comparitors. These drawers do not obey that rule and instead they do nothing with a comparator. my proposal is that the drawers should emit an overall signal of how full they are in any slots (so a 4 slot drawer has to have all 4 slots filled to output 15 power) and the redstone upgrade should let you choose both the slot and the side it outputs that particular slot's fullness. This both keeps the drawer inline with vanilla inventory behavior whilst keeping the redstone upgrade relevant.