I've used Safety Seal for a few years now in my fleet of ground transportation vehicles. They're much more reliable than the cheap Walmart/parts store kits. The tools are well made and the strings are much fatter and sticky so they will seal bigger holes. They always last the remaining life of the tire, no porblem.
The key is to not "rasp" the hole in the tire unless absolutely necessary. Put some wax on the rasp and insert it into the hole then move the handle from side to side and in all directions to stretch the hole open. Have the insertion tool ready with a dab of wax on the tip, pull the rasp out and quickly insert the plug. The rubber around the hole will shrink back making a perfect seal. Never use rubber cement!
I've had plugs hold for years. Only one time I can think of that one wouldn't hold air, always used rubber cement too. And yes the Wal-Mart and parts store kits use junk tools, the handles would always separate when trying to do the job.