[ludbb]

I believe the reference is coming from http://docs.ethereum-alarm-clock.com/en/latest/overview.html...:

Guarantees - Will the call happen?

There are no guarantees that your function will be called. The design of this service is meant to provide the proper motivation for calls to be executed, but it is entirely possible that certain calls will be missed due to unforseen circumstances.

[avmich]

Strictly speaking, there is no such a thing as "guarantee". Your alarm clock may malfunction in an important moment. An asteroid can suddenly wipe civilization with all insurance providers (now that's an idea...) So you - in a strict sense of a word - always have to estimate and compare probabilities.