A motorized ball of whirling, razor-sharp, eviscerating death, designed to plough through enemy lines with hellish vigour.
The Doom-Flayer is a fast, armour-piercing, anti-infantry shredder its hard not to find a use for such a mount. Notably, both the units who can get it actually lose damage and melee attack/defence (it is technically a chariot unit). So perhaps if you expect your character to be engaged in duels or targeted heavily by enemy characters, don't bring this mount. Even in that circumstance though, the speed of the flayer can often allow you to avoid danger.