My experience, after patching hundreds upon hundreds of tubes for local nonprofits (and my own re-use):
Surface prep is 95% of success. If I'm patching at home, I use a dremel with a sanding drum attachment to get a good clean nothing-but-tube-rubber patch area about 25% larger than the expected patch. Turns a tedious chore into seconds of easy work.
I use only Rema cement. I buy the 4-pack of 7 gram tubes every so often - if there's a long hiatus between patching sessions, I only potentially lose what's left in the currently open tube. NOTE: the newer Rema cement tubes say the formula is now a "no-wait" one and the patch can be applied immediately - I still wait 3 minutes or so out of habit.
I've used non-Rema patches with Rema cement with good success, but using Rema (or any) patch with non-Rema cement sees an unacceptably higher failure rate.
Oval Rema patches have an amazing ability to fix long cuts / blowouts / bigger holes, but aren't 100% foolproof . Don't ask me how I know this.
The bigger frustration is now they're nearly 50¢ each on Amazon, so wasting an oval is an annoyance.