Eva, I agree that if the "unsourced family tree" is for events within the lifetime of the contributor, or perhaps the lifetime of someone the contributor could have known, then it's a valid source; except that when it's simply documented as "unsourced family tree," how are you or I to know whether the creator of that tree had personal knowledge of the events, or had heard about them from the lips of someone who did?
In my own case, there are a very few places where I have relied on otherwise unsourced materials from family members. But in every case, I've documented this as "family history notes created by," followed by the name of the person who created the document. In a few other cases, I've used something like "event related by," followed by the name of the person who gave me the oral information. But in both situations, most of the things I've documented in these ways have been supplemental to the basic BMD information, for which I have always at least tried to find documentary sources.
In so many cases, "unsourced family tree" profiles could have been tremendously improved with only a very little effort on the part of the profile creator. I'm currently working on a profile for a man with the dates 1827-1899. The profile was created in 2020 with "unsourced family tree" as its lone source. But there exists for this man an Ancestry.com profile with no fewer than nine citations—six vital records, a city directory, a census, and a published family history. How much better the WikiTree profile would have been if the creator had investigated those sources and cited them here—or at the very least, included a reference to this well-sourced Ancestry profile (which I have now added, along with other sources)? I'm assuming, of course, that the Ancestry profile pre-dates the WikiTree one, but even if it does not, a little time searching with FamilySearch or Google would have unearthed the same sources that the Ancestry profile creator used.