But that IS the point. We don't know. It isn't studied -- cannot be studied ethically.
It is presumed to be painless based on unrelated case studies. And so people are proudly and confidently stepping forward to say "ignore the situations where it causes apparent pain and distress (animal examples), we'll just use very different industrial accidents where we THINK it maybe was painless but have no way to know and will use that to declare it is painless."
Meanwhile this guy struggled to live for over 20 minutes tied to a gurney.
You have a belief without evidence. You have to prove it. And we both know it is not going to happen because the research doesn't exist and would be unethical.