100% of the tires on all the trucks at any roofing company have probably >5 plugs in them at any given time. I have never heard of a plug releasing that took a hold in a tire. If it's not leaking after a day, it's permanent, and we frequently end up using cheap ones from wal mart.
That's considering that we frequently overload our tear off trucks and drive them on the highway every day, and my family has been roofing contractors for three generations now.
Nothing wrong with tire plugs, just keep a close eye on it for a while if it takes more than one or two or if it's in a radial sidewall. Works a whole bunch better if you let the air out while it's drying BTW. If you don't believe me, try putting some effort in to the prep and installing one on a junk tire. Come back a day later and try getting that sucker out. The rubber's not the structure of the tire, a tire is just a composite that uses cording/belting for reinforcement. The rubber only serves to be airtight. There's less surface area to fail with a plug than there is with a patch, and the plug has pressure from the tire itself squeezing it and increasing friction.