"Universal" anything is always a disaster...
"UWP apps aren't designed for desktop, tablet or phone, but can adapt to each display size. Predictably, this lowest common denominator approach has been an aesthetic disaster,"
Yup. Over the decades, I've been involved to various extents in a number of efforts to develop a "Universal Something-or-Other. A "universal board set" for use in products with different requirements, a "universal table-driven application generator" that was going to... I'm not sure what... [I only went to a few project reviews for that one]. A universal device API/library that would allow application writers to run the same code on Windows or on various RTOSes. A "universal transmitter OS" that would [blah, blah, blah], etc.
All were complete and utter disasters. They did nothing well and often nothing at all.
Development often went on for years and delivered nothing. When they did deliver something the performance was awful, the resource usage was huge, and using them didn't save any development time or reduce bugs.
They _always_ got their asses kicked (in any and all measures) by application-specific designs.
But people keep chasing the holy grail...