I don't think there is any legitimate research on the subject and the opinions are unreliable. Car tires have a good history with plugs and patches but car tires are built differently and flex differently. The sidewall and tread are defined so flexing is defined.
Sidewall patches on a car tire are a bad idea.
Since a motorcycle leans there isn't a clear distinction between sidewall and tread. How much flexing depends on an infinite combination of factors. Tire stiffness, load, speed, angle of lean, size of tire, design, pressure, temperature etc.
Then there is the riding situation. If all the rider does is a Timmie's run at city speeds I see little risk if the nail is central. People that race have a higher risk factor.
People that tour have to think about having a plug let go a thousand miles from home and the consequences. They also have to think about using a plug when they pick up a nail a thousand miles from home.
I think of a plug being squeezed and flexed like someone trying to pop a zit. If I could see inside the tire and confirm the plug wasn't moving out I wouldn't have a problem with them.
Invention needed: A multi coloured plug that would show red if it was coming out. Go for it.
The simple answer is that there is no simple answer.