Disclaimer: This is my own personal experience with my own personal devices. There may truly be software glitches out there that cause alarm problems with your particular device, however this is my story!
I Have been using my iPhone(3gs, 4, 5, and 6) alarm as my sole alarm since 2010. I have only had an issue with my alarm "not making sound" and failing to wake me up twice. Both times, coincidentally, I had a few beers the night before and stayed up WAY too late. When I upgrade to the newest model, my wife gets my old phone(she doesn't want the latest and greatest until all of the bugs are gone). All of a sudden, the the alarm that has faithfully awakened me for the past 2 years "fails to make a sound" about once a week. This has occurred for her on IOS 6, 7, and 8. She always sleeps through at least the first minute of her alarm (at which point I either move the phone right next to her ear or wake her up myself), and the phantom alarm silence only seems to occur when I'm not within earshot of her phone during the alarm time. I cannot help but take these complaints with a grain of salt. The iPhone is NOT a LOUD alarm clock. If you are a light sleeper you should be fine. If you are sleep deprived or a perpetual snoozer, perhaps you should stick to air-raid sirens played over a loudspeaker. It grinds my gears when people blame their technology for the "ID10T" errors committed by the user. If you are experiencing this issue, go to to an Apple Store and get trained on how to use your device, or attach your device to an amplified speaker on the other side of the room.
Once again, this is my experience, so please feel free to take it with the the same grain of salt that I take the alarm failure stories. Apple does have quite a few bugs in their software, but I feel this particular one may be caused predominately by human interference. Sorry if I offended anyone, that was not my intention. I just wanted to share my anneceltal experience to encourage considering alternate causation for this issue.