Hello Simon, this is Emily.
If your C Drive is running low on space, or if you have moved your user profile folder to a drive that's not C:, or if your recovery partiton or if the image reset needs is corrupted, it can usually lead to reset failure. Then yes, you need to use the bootable USB drive to do a clean install.
Typically this shouldn't happen again after the clean install, but it is also not guaranteed. The reset process relies on your exisiting system, so if something is not right with the current system, it can also affect the reset quality. But the USB drive route relies on external resources, the files you download from Microsoft, so if the current system is all messed up, it won't affect the USB drive, hence it can reinstall computer no problem. I hope that makes sense.