Maybe it's time I waded into the conversation. Let's start with what the Rubber Manufacturers Association (RMA) says about tire repairs:
https://www.rma.org/publications/tire_service_professionals/index.cfm?PublicationID=11303
This is a wall chart that shows the proper repair procedure for tires as determined by a concensus of representatives from tire manufacturers, tire dealers, and tire repair materials manufacturers. It represents the best knowledge available.
3 things:
1) While the sidewall is considered to be non-repairable, so is the outermost rib.
2) They specifically label "plug only" repairs as not recommended.
3) They specifically label "patch only" repairs as not recommended.
Having sat on this committee and listened (and contributed) to the discussions, I can tell you the rationale behind each point:
Plugs: There is a certain failure rate for plugs (defined by leaking) and it is orders of magnitudes higher than the leakage rate of new tires (the standard by which all repair materials and procedures are judged). My own experience bares this out as new tires have leakage rates measured in parts per million, while plugs are on the order of 10%.
Plugs also do not provide support for the damaged area. A patch "bridges" the structure of a tire, which reduces the stress concentration caused by the hole.
Sidewall: The motion a sidewall goes through is pretty complex and patch repairs do not adhere well to the sidewall. Plus there is the stress concentration issue.
Outermost rib: This is the location of the belt edges - the most highly stressed area in the tire. Tire engineers do not want anything to disturb this area - that pesky stress concentration thing - since this is the area where tread separations start - that pesky Firestone problem!
Patch only: Patches do not fill the injured area. Contaminants can enter the area and since there are steel belts, cause rust. Very bad!
The key to understanding this is the failure rate. While I wasn't there, I was told there was a long, acromonious discussion about what should be the standard about repiars - and it boiled down to: All repair materials and repair procedures should result in a failure rate that is comparable to a new tire's failure rate. In other words, a repaired tire should perform as good as a new one. While is is truly unachieveable, the failure rates should be in the same vicinity and not orders of magnitudes different.
Just for the record: Properly repaired tires fail about 2 to 3 times more often that unrepaired tires - and that was judged to be an acceptably close.
I am sure there will be folks who will say "I've never had a plug leak", or "I've run thousands of miles with no problems - and I am sure they aren't fabricating their experience.
The RMA recommendations are based on samples sizes that are way beyond what a single individual can experience - and they are indeed based on real world experiences. The repair manufacturers are present for many of the discussions (They ARE members of the RMA), so it isn't like it is all the tire mnaufacturers dictating what to do.
On a personal note, I have used plugs and I've had a few leak. But I understand the risk involved and I am willing to accept that. If a person feels - like I do - that they are willing to accept this risk, great!
The purpose of this post is to inform everyone about what the risk levels are and allow everyone to make an informed decision.