The biggest advantage vs a bigger desktop unit is the bowl size and the unit size. It allows you to save more bud, and it's more easily hidden or packed away.
The UD is a traditional log vape, the nano is a log vape with an ac element which allows it to heat much more quickly without potentially damaging the element while rising the temp to the max to get it to vaping temps.
The size difference between the two is marginal although the twig is smaller. The UD can more easily be used with any 12V power like in a car, or anywhere with 12VDC, but it doesn't have in-built temperature control. YOu would need to get a Variable Voltage Power Source.
I have a HI which has replaced my Solo when i'm at a friends house, it heats so quickly that i don't even take that into account.. while i'm preparing my stem, i plug it in and it's ready to go when i am.
But has it's been said, there is a waiting list so, the UD and the Nano are better options if you want it soon.
The UD also has the art factor, and i find them much more beautiful than the nano, but that might not be important to you.