I've been told the concept of scaling a UI is akin to the concept of "write once, run anywhere".

That is, it works fine in theory. It just hasn't ever worked as advertised in reality. And trying has often generated more pain than it's saved.

It's worth noting that much of modern web design moved to a place of fixed minimum widths and white-space taking up the remainder. Responsive design promises to change that a bit toward scaling UI, but thus far is just a mechanism by which designers can incorporate a few hand-tuned minimum-width variants.