alan, your mechanic does not know what he / she is talking about and your difficulty in patching may stem from user error or the fact that in very few rare cases. the surface of the tyre has been compromised so that the patch cannot vulcanize properly.
If you know how to properly patch a tube with cement, and not some pre-glued type, the cement vulcanizes the patch to the tube and the patched area will be stronger than the tube.
When patching a tube with vulcanizing cement the glue need to be allowed to dry before applying the patch, if you do it wet the patch will not stick.
I have serviced tyres that have had multiple patches on the tube and these may have been in place for decades and they have stayed put and not developed leaks.
Pre-glued "quick"patches are not as good as they do not vulcanize themselves to the tyre... vulcanization is a chemical process whereby the patch bonds to the tube at a molecular level and if done right, you will not be able to separate the patch from the tube after a repair as they will be as one.