For many people, yes. It's also a very common belief that one and the same state of the environment is bad if traceable to human actions, but good otherwise.

The working premise in many environmental discussions is that humans carry an inherent moral taint and whatever they do creates a problem that needs to be corrected, because it came from an evil source, regardless of whether the resulting state of the world is good or bad.

I don’t think that is what people are suggesting. Rather, man made is unknown. At least the long term affects are. We can confidently say that natural chemicals, even if they’re bad for you, likely won’t have a giant, unexpected impact on the world. Whereas there are a lot of man-made chemicals that are likely harmless. But less is know about the long term affects of injecting it in to all of earth’s various natural systems.

Often, yes: novel compounds can have side effects which take years to understand (e.g. DDT‘a impact on bird populations lasted past the point where its effectiveness was rapidly tapering) and depending on what characteristics something was picked for you can end up with something which doesn’t biodegrade and is thus a long-term problem if it doesn’t turn out to be harmless.

It’s a good question and in my opinion, it depends entirely on the compound.

But that wasn’t the point of my comment. I am calling out people being pedantic and nitpicky just to argue, instead of recognizing the very obvious intent of the great-grand parent’s comment and debating something with merits, like whether man made chemicals are inherently bad.