Still not clear - how would a mushroom plug fail?
It's not like it can get sucked inside and because of the cap it cannot be expelled.
When pulled tight the cap is flat against the inside completely covering the hole. The stem has a diameter about 1/4" which plugs a pretty freaking big hole and is of softish rubber so it conforms to the hole.
Every Stop-and-Stay mushroom plug I have installed in a motorcycle tire has failed. They've done OK in car tires.
Basically, the heads of the plugs end up bouncing around the inside of the tire. As the tire flexes, the soft rubber plug gets sliced in two and the inside "mushroom head" part pops out.
To be fair, I haven't tried them in radial motorcycle tires. But they failed in every instance when I used them in a bias-ply motorcycle tire. After I went through six plugs in a simple nail hole to make it 50 miles home, I switched to a "sticky string" kit.
If your experience with these plugs has been different, great.
They have failed repeatedly for me and for many others, and there are much more reliable, not to mention cheaper and easier, alternatives for making puncture repairs available just about anywhere.
I carry a Nealey kit now -- it's basically a "sticky string" type of plug that's installed a little differently than most. They also are made with a slightly different compound that doesn't require glue. When you install it properly you end up with a knot on the inside of the tire that retains the plug. They're very versatile, and they even come in a nice little tube that tucks into your tool kit.
Nealey Tire Repair Kit - Repair Kits
(the mini kit is perfect for motorcycles and cars)
I've never once had a problem with the Nealey plugs or with the many "sticky string" plugs I've installed.