It's the user who's mobile, moreso than the device, and users will turn to the best screen available when they want to get things done. Will your product work on that screen, whatever size, shape, or capabilities it offers?
Can a single information architecture serve multiple device types? How do you design simultaneously for portrait and landscape orientations, and multiple device types, sizes, and screen resolutions? How are digital experiences like products and how are they like services?
And how do you design experiences for users that may be moving from device to device and moving their data from app to app from one person to another?
These challenges demand a holistic approach to user experience design that must break out of the boundaries of a single particular device or even a single application, and embrace users where they are and when they want access to the service inside the product.
Holistic UX design starts with exploring and understanding user journeys in the larger ecosystem, and then works from back to the front, building a solid foundation in the platform layer before developing any user interface.