To complement the above thoughts, it may also be instructive to consider what strengths UA may boast beyond those found in the GoldBoxes.
The GoldBoxes were team efforts, whereas most UA mods are the creations of indivdual authors. For UA, this strength is maximized as individual authors find their "voice." A "voice" unique to one author is demonstrated in the stories they tell and how they tell them. In telling a story, even if it is not original to the mod author, there are many choices to be made, from details and emphasis in text (even how much text), to what art to use where. The more these choices reflect an individual author's tastes, the more they have their own recognizable "voice."
It is not unusual for UAers to have favorite mod authors on the basis of this strength. My personal favorite mod author is Steve Gustafson, due to his prevalent wit and whimsy. I wish he was still active, writing more mods for me to play.
Another strength UA may boast is the creative freedom it gives its authors. The GoldBoxes were limited to a particular style. The writers and artists adhered to certain rules, guided by the expectations of their audience, as well as those of TSR. UA has no such creative shackles, leaving its authors free to indulge their own tastes, and free to experiment (even wildly experiment).
Gore, sexuality, taboo subjects, foul language, religion, politics, and more, that you would never see in a GoldBox game, can already be found in certain existing mods. Authors can be deliberately offensive, taking their own chances on possibly alienating their audience.
Moreover, authors may stray from the straight, epic adventure styles of the GoldBoxes to fashion comedies, love stories, mysteries, horror stories, metaphors, etc., of any mixture, and of any length.
So, while the basic GoldBox engine elements of UA may grow stale, there is still much room to add freshness to any new mod.