Statistically, it is guaranteed, if we're taking the averages, and you think a willful evil creature is going to continue to happily serve after it's been subject to Planar Binding? That seems ... generous.There is a grotesque look.
The Bastion is extremely manipulative, evil and powerful.
Nightmares are intelligent creatures that serve evil creatures.
Well, as it is average, the creature remains as a mount.
It's is also a not granteed dispell.
So, are you falling, or are you riding an unwilling mount? Those are your choices.