I've been plugging my tires for the past 30 years. They have been reliable and have lasted until it was time to retire the tires, if that makes sense.
I always bring plugs with me on the road and have plugged flat tires in campgrounds while on long trips. This has saved me a lot of time and trouble.
I have only had to replace two plugs in all that time: both were on a utility trailer that came with the cheapest tires I've ever seen. I knew when I was inserting the plugs that they may not hold, because the cheap tire was so thin I could push the plugs in much easier than normal trailer or truck tires. Sure enough, they began leaking about a pound of air or so a week.
Other than those cheap utility trailer tires, plugs have worked flawlessly for me.