> the papers struggle to get good data
Note also that I'm specifically talking about inequality in isolation ("as long as it's caused by the rich getting richer and not by the poor getting poorer"), which I would argue is even harder to study.
It would be completely unsurprising to learn high inequality is correlated with higher rates of poverty, which is correlated with all sorts of other negative causes and effects. I don't know how you could control for that well enough to be able to convincingly claim that inequality itself is what's causing the problem and not the poverty (or the underlying causes thereof).
I would also acknowledge higher inequality certainly makes people envious, and that that probably has some negative societal effects. But I don't know I'd go so far as to count making someone envious as "materially harming" them even if there are other significant downstream effects to that.
I'll admit there's a lot more reading up I could do on this, but it would take a lot to convince me of the idea that making one group of people wealthier without hurting anyone else's finances is a net negative on society merely because it "increases inequality".