The word 'un-failingness' implies consistency, reliability, and trustworthiness. As such, its antonyms would be words expressing the opposite of these characteristics. Some antonyms for 'un-failingness' could include unreliability, inconsistency, unpredictability, inconstancy, unreliableness, and instability. These antonyms suggest a lack of dependability, inconsistency, and untrustworthiness. They imply that something or someone cannot be counted on to perform, deliver or fulfill expectations as promised or expected. While being 'un-failing' is seen as an admirable trait, its antonyms reveal the negative aspects of the opposite behavior.