Over the last 25 years I've plugged and or patched a half-dozen motorcycle tires (or tubes) with no problems. Only once have I had a failure after patching a tube, and that was because I pinched the tube when reinstalling the tire. I've never had a plug fail after plugging a tire.
I do have several guidelines for what I consider to be safe/unsafe for plugging:
1. The hole must be pluggable with one cord type plug. If it's larger than that the tire is either replaced, gets an inside cord-type plug/patch, or gets the plug/patch and a tube.
2. The hole cannot be more than an inch off of either side of centerline or I (a) install a tube, or (be) replace the tire
3. A hole or cut in the sidewall gets an immediate tire replacement
Patches on tubes are a creature all their own. The tube I mentioned above ended up with 3 patches on it. I finished the last half of a 5,500 mile trip running that tube, and replaced it only when I replaced the tires a year later.
I carry a Slime tire kit and compressor, and an extra set of cord type plugs from Autozone.
Current ride: 2001 GL1800; Gone away: 5 BMW's, 4 Honda's, 3 Suzuki's
"The object of life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane." -Marcus Aurelius