I'd done more than i can count back when I was a dealer tech, have a shop now but don't really do this sort of thing. I was always weary but still trusted them and used them myself. September of 2018 I was finally able to put any doubts to rest for good.
We borrowed my dads 08 Boxster and were camping around northeast Canada for 2 weeks, my parents did similar for their honeymoon in a Fiat X1/9 40 years earlier. Every couple nights we stayed in an airbnb if that was more convenient than camping, one was down a very rocky road and sure enough we slashed a sidewall on the way in. This was saturday AM of memorial day weekend so virtually no one was open.
Canadian tire didn't have a single tire in the size we needed in the country but they could "overnight" night one on tuesday and hope to have it by thursday for an extra 100-150 bucks on top of a tire that was going to cost double what it would in the US.
After exhausting every other option we decided to try a plug ( and after service station we stopped at finally found one to sell us).. one didn't hold.. screw it throw a second in, OMG it is holding. Drove a few miles, checked, good.. repeat a half dozen times. Went for a hike (down a dirt road), checked, all good.. back to airbnb and to bed, wake up, held pressure overnight!
Ended up doing 2k miles on the double sidewall plug before we could get to the Porsche dealer in Portland who was able to order us 2 tires at a reasonable price.