The whole bone mount can be taken off so might be cleaner than a Garmin out-front mount. Same if you're using the standard mounts. Bone might be advantageous if you plan on changing phones any time soon - it fits different sizes whereas your custom case will probably not.
Yeah, I do trainer road through it. And it's nicer than my fire tablet that needs to be returned....
No window sill.
Will cost up case +mount Vs bone.