After the second time a hotel missed my wake-up call (probably 3-4 years ago), I don't bother with them anymore and just use my cell phone. It is not risk free either...since the phone may run out of power, or that I may have set the alarm for a different time. However, the last few smart phone I have is great in that it will even "ring" when I have it on low volume, or even if the phone is powered off). Also, it is consistent with time zone changes (it always follow local time).

Also, it is easier to change/cancel the alarm on my phone then to call and cancel a wake-up call, especially if you are staying in a foreign country.

In the end, you're relying on a human to write down the wake-up call on paper, or input the program into the computer. He is probably doing 10 other things at the same time, so he/she may forget. Also, the computer system that is doing the calling may malfunction, be late, or somehow, someone erased/changed your call by accident. Also, it is possible for you to knock the phone off the hook while throwing the redundant pillows off the bed. All these things have happened to me one time or another... :-)