Since I consider myself an historian as well as a genealogist, I am not bothered by the presence of a Uther Pendragon on WikiTree, but that is because I am not primarily concerned with the tree, but with the history that the tree connects. And in Uther's case, Uther the legend is a part of history. If he was claimed by Welsh Nationalism, that is part of history. If he inspired poets, that is part of history. As a legendary character, he deserves a writeup somewhere. But as a legendary character, he did not sire children; legends do not beget. That is why I'm a strong advocate of there being a profile for Uther, one that is marked with { {questionable} }, one that has a beautiful narrative on what the legend is, what kernels of fact it might contain, how the legend came about, what it has inspired, and how it is used even today -- and one whose links to any parents, spouses, siblings or children have been removed.
Now, in the case of Uther, the { { questionable } } marking is already there, but the profile needs more research as to the legend, and how it cannot be factual. And then the WikiTree-polite thing to do is to post a note stating that no documentation can be found for any of the relationships shown, and that you would be delighted if persuasive documentaiton of relationships can be posted, but otherwise in the interest of accuracy, you propose to cut them. And then a week later, do it. And if there is static, practice the proverbial "Stop, Drop and Roll" and figure you've done your bit and put it in the hands of a leader!
Actually, the bigger problem is not Uther, who has a status in legend, but the next generations after Uther, which are names of totally unknown and unproven people whose only claim to existence is to link Uther with a famous descendent. Those profiles DO need to claim their spot in oblivion, although even there, there could be a mention in the Uther narrative that says, "a part of the legend is a line of descent to Rhodri Mawr which is presented below for informational purposes...No documentation whatsoever has been found to support the existence of any of these people."