Tafsir of Al-Qamar 54:46

Rather, the Hour is their appointment—meaning, this is not the entirety of their punishment; rather, the Hour is the appointment for their torment, and this [worldly punishment] is merely one of its precursors.

And the Hour is more disastrous—meaning, it is the greatest calamity, which is the abominable, horrific affair from which there is no guidance to escape.

And more bitter—in taste. This is a metaphor for its difficulty upon the soul. It has been said that it means "more intense," but that is not as strong. The Hour is explicitly named here, rather than using a pronoun, to amplify its terror.