Plugs may be OK, but don't let anybody get themselves killed thinking they are universally safe.
Remember, freeway speeds are now from 65-80 with a margin exceeding that. In the weather here where temperatures can exceed 120F and asphalt and tire temps are greater, a plug might just get you killed.
I keep a plug kit, glue, the correct tools and a capable electric pump in order to get to the Discount Tire, if necessary. It has saved my ass. These guys remove the tire, scrub it with an abrasive rotary tool, and then install a mushroom shaped plug that will not come out. They then rebalance. FREE! I was just there yesterday for just this.
If you put a plug in a V or Z rated tire and use it to that rating, may as well play Russian Roulette with all 6 chambers loaded and hope for squibs.....
Same for motorcycle tires.
That is all.