Meh. How is making a machine that administers a fluid intravenously complicated?
I take it you arent in the medical industry and have never had to give an IV based on this statement. As I said to those who are knowledgable on this the answer is many ways:
1) Finding a vein, this can be hard on many people. In the hospital it isnt uncommon that an expert wont be able to find a vein on either arm or even the hands.
2) Once a vein is found it may not work, for example it may be a collapsed vein and wont take the drug.
3) If there is damage to the vein it may leak into surrounding tissue causing a less than lethal dose, or cause the death to be prolonged
4) if you miss the vein you may accidentally inject into the muscle or fat, again causing a less than lethal dose or prolonged death
5) Since the lethal injection is a multi-stage dose you have multiple chances for error that can cause suffering. For example a sedative is administered first, then the lethal injections. If the first stage breaks and the second succeeds it can lead to suffering.
6) Because of the increased complexity drug interactions can prevent or effect the first stage and thus cause suffering int he second stage.
7) You can have something clog up the system or a motor break or something, since its an orcastra of events this can lead to all sorts of suffering or issues
All of the above more or less doesnt apply to N2.