I dunno, but I found out today I have had like a 3/4" thick stick buried about an inch below the surface between the treads in one of my tires.
Yay multiple steel belts! Never knew....never lost any air and I have been running it at about 4psi on the trails and airing back up to 25psi on the pavement.
I have plugged lots of tires in the tread area on different vehicles i've owned, I have even had some that required two plugs in the same hole. I have not had any fail. I would only be worried about failure on a plug used in the sidewall of a tire.
they use a raw rubber, and some neat glue. plug it home, take it for a drive to warm the tire up, and call it good. that plug almost becomes part of the tire, you're not going to pull one out.