Unless they fuck up the system and don't remove the CO2 properly, or the person panics and holds their breath, or any number of other things that could possibly go wrong.
If I was dying of cancer and a doctor was managing the setup, yeah it would be great. But some amateur putting someone to death with Home Depot leftovers because medical professionals won't participate in the death penalty due to having ethics means that it will probably go as wrong as their previous attempt to put this guy to death.