https://www.youtube.com/watch?v=mrBPW-ciQCY

like shown in the video, if you have a hopper minecart slowly losing items, due to a hopper in this case, and a detector rail and a comparator reading the items in the hopper minecart, it won't update reliably. It can clearly be seen that all hopper minecarts contain the exact same amount of items at the start. They will unload at the same time, yet the lamps will turn off slightly after each other, thus making it quite unreliable.