I'm not saying I'm right, as I'm not a biologist and this is just stuff I've seen (oft repeated) on the Internet, but:

A search on Google Scholar shows multiple publications exploring the adverse effects on sweeteners on gut microbiome:

https://scholar.google.com/scholar?q=artificial+sweeteners+e...

I've definitely read that it may play a role in decoupling the taste of sweet with satisfaction. That in turn may lead people to ultimately eat more before feeling satisfied, which is bad.

Also, I think there might actually be some links to type II diabetes, but I'm skeptical of causation on that one. It's not implausible, but fully correcting for people already trying to reduce sugar intake sounds essentially impossible in that case.