Objectively, the guillotine has a lot going for it compared to lethal injection, the gas chamber or electrocution. It is quick and sure, and probably painless. There have been a few botched guillotinings in history, mostly from the blade getting stuck in the track and the like. This can easily be avoided and the machine is certainly easier to manage than a cocktail of lethal drugs, not to mention infinitely more predictable in its immediate effects.
It isn’t worth a damn at the real purpose of these supposedly more humane, bloodless, and antiseptic methods of killing, though. That is to soften the psychological impact and disguise the event as something other than what it is.
Among others, Germany, Sweden, and France all used the guillotine until well into the twentieth century (France until 1977). All have long since abolished capital punishment. I wonder if the method made final abolition easier to sell.