A tire used on the road; If nearly wore out, and I don't plan to cap, I would plug and hope I can get to a shop. A good tire? Not happening until I have it broke down to see inside.
The risk is the nail or whatever goes in thru the tread at angle, or bends at the belts. Now it takes some force to shove the cleaning tool in, and it is more rigid than the nail, it will likely go straight in. So now you have 2 holes in the air chamber. You stick the plug in, and it seals one hole in the air chamber, and the hole in tread. At this point, if the gods are smiling at you, air it up, and the tread or sidewall bulges out, and all you have done is change a repairable tire to junk. But many times that bulge doesn't form until hiway speed and temp. The first indication you have a problem is BANG!
IMHO plugging a tire is like rolling the dice, except when I lay money on the **** table I know what I'm putting down, and know the odds of picking up my money twice or not at all. When betting on the plug, all I can pick up is the little dab it cost to get it fixed right, but a blown tire? What is the limit on what that can cost?
BTW, a tire that the last cap is about wore out, I have been known to pull a nail, put patch cement on a screw, and drive it in the hole. The screw is more likely to follow the hole, and will leak much slower than the nail...
Wow! did not know you could not discuss the game played with galloping dominoes...