I patch several hundred tubes a year for four different nonprofits. Doing it in bulk saves a lot of money for these organizations with now that wholesale tube prices are up above $5 each.

I agree that proper surface prep and the correct tools & parts are very important for success. When working in bulk, I use a battery powered dremel with a sanding drum to get a clean roughened patch area (going 20-40% larger to be safe) in seconds, with all bumps, seams, and ridges obliterated. Now this isn't typically an option on tour (the tool doesn't weigh much at all, but it's bulky), but for shop or home patching it's a huge timesaver & greatly improves the likelihood for success.

I also only use genuine Rema cement and typically use only Rema patches. I've tried off-brand cement & patches with much lower success rates, as the cheap cements just don't create as good a bond and can stay "not set" for a while, allowing peel-off. I have successfully used non-Rema patches with Rema cement when a Rema patch won't cover the hole properly, but I prefer the Tip Top whenever practical. I've encountered tubes patched with old butyl tube casing instead of a patch, and all too often it's an easy task to peel that patch away with bare fingers, let alone what air pressure can do with much greater force.

All this being said, patching a slit or tear is tough even when the prep, patch, and cement are exactly right - the tube still really wants to continue separating along that plane of weakness. Sometimes it holds together great with a good Rema oval patch... and sometimes it doesn't. The only patch locations with lower success rates are ones where the hole is less than 1/2 patch diameter away from the valve.