Amazing how people think they are experts and claim things cant possibly have any effect. Talk of ethernet protocalls, snake oil, and rants about people imagining things are clear signs of typical lack of college level electrical engineering from “experts” who know just enough to be dangerous.
Ethernet is a tightly specified interface with a strict characteristic impedance. This is fundamental in transmission line theory.
The UEF units are what is called a “stub” in high speed electrical transmission line designs and iIremember learning about them 45 years ago when I was active in amateur radio . They are really nothing novel even for ethernet except for whatever electrostatic effects their UEF compound may have.
There were Japanese sites discussing stubs cut from a sacrificed ethernet cable and numerous threads are around discussing the implementation and tuning as well.
I have tuned stubs in all my unused ports except 1 and recently purchased one of the Synergistic units to see what it brought to the table. Like others results it gave the impression of greater bass and a micro textural enhancement. Not sure i like the bass actually. To me it seems plump or wooly. I did not remove all of my stubs from my ports and test just the UEF device though and maybe thats next.