It is propaganda, read through this wiki[1] and see how far people have gone to create propaganda against it. It's one of the most researched ingredients by the FDA.
It seems there are a lot of studies where the author starts with the conviction that Aspartame is "bad", and designs a study to "prove" it. For example, "what if we give these lab rats a massively excessive dose of Aspartame every day for a year, and then check if that harmed them"
You are right that my comment as written did not do a good job of explaining what it is about these studies that seems fishy to me. So I thank you for responding to it and making me examine my opinion in more detail.
Someone else already responded to you and provided a much better explanation.
There is always something that rubs me the wrong way about these studies that set out to prove that artificial sweeteners are bad. It's intuitively appealing: it seems too easy that we can just replace sugar with these harmless chemicals and then it tastes mostly the same but without the calories or bad metabolic effects of sugar. But artificial sweeteners have been studied for decades, especially Aspartame, and as far as I know they appear to be harmless, outside of the Gwyneth Paltrow universe. So it seems to me that some people still have an axe to grind with these artificial sweeteners, and try to bend science to their will.