Fair question, I was wondering that too
If it works like most other things that run off firmware, it doesnt matter if the flash fails, you can re-write it again, presumably from the backup they took of your ECU before starting to flash it. This is how it works for things like computers and phones, so I dont see this working any differently. If the ECU was bricked for some reason, I would be expecting RS Tuning to be picking up the bill for a new ECU, but I think it highly unlikely they would have stock of the different types of ECU in house. I dont think I have ever heard of an ECU being bricked in all my time of following remapping though, so cant see this being a problem.
If worst comes to worst they can debug it on the bench or whip the chip off and replace/reprogramme it off the ECU.
R