Is it OK to install a Uhing assembly without having the mounting surface of the drive facing upwards?

Yes but it's not the ideal set-up. When a Uhing assembly is placed in its side, the bearings are subjected to different forces than when the assembly is upright. This could reduce the thrust capacity of the drive causing it to skip or causing the bearings to wear down faster. If a Uhing assembly is installed sideways, then a linear slide load carrier is recommended to counter torque and other overturning moments.