I think, in the absence of clove oil (if you can't find it or just don't have access to it for any reason at the time), then freezing would be the next best alternative, but I still think freezing is worse than clove oil. Simply because it takes longer. I guarantee that a fish in the freezer takes more than a minute for their systems to shut down, and even if they don't know they're dying the way we do, their instincts know. There is also a certain amount of pain in freezing and it lasts longer than a few minutes. Have you ever been really cold? The likelihood is, you were no where near hypothermia, but you probably weren't comfortable. Imagine how much worse it would get before your body actually started to shut down?
Clove oil does not always kill, true. That is why I recommend doing something else to make sure of death, but the clove oil makes sure that the fish does not feel what else you have to do. With my loach, I severed his spine to make sure. He did not even twitch, so I *know* he couldn't feel anything, if he was even still alive. It's like anesthesia. Even in the human world, lethal drugs are considered the most humane method of execution.
That's just my opinion, but if I ever have to euthanize a fish again, I will use clove oil. Normally, I do everything in my power to save them, so they go on their own, but if they are obviously in pain, or there is nothing I can do, such as the case of my loach, I will euthanize.