I plug shoulders in large part because it drives certain people up the wall. I'd say maybe half the time they last the life of the tire. Frequently they're finicky and unreliable. Less often than that they start a tread separation bubble. In either latter case I just trash the tire since that's easy enough.

Yeah a proper internal glue on patch would likely perform better in the shoulder but ain't nobody got time for that, that's like 90% of the work of changing the tire.