The unicode-math package re-defines ellipsis to be the ellipsis character in whatever font you're using, instead of spaced dots. This looks bad in my opinion, and violates the guidance on ellipses in every style guide I own. A solution for \ldots is provided here: variations-in-ldots. But that solution does not fix \cdots, \vdots, or \ddots.
I managed to find a solution for \cdots that works in this MWE, but seems to fail in other instances that I haven't figured out a reliable way to reproduce.
And I can't figure out a solution for \vdots or \ddots at all.
\documentclass{article}
\usepackage{unicode-math}
\AtBeginDocument{
% From https://tex.stackexchange.com/questions/120627/variations-in-ldots
\renewcommand\mathellipsis{\mathinner{\ldotp\ldotp\ldotp}}
\renewcommand\cdots{\mathinner{\cdotp\cdotp\cdotp}} % sometimes works
%\renewcommand\vdots{\mathinner{\vdotp\vdotp\vdotp}} % doesn't work
%\renewcommand\ddots{\mathinner{\ddotp\ddotp\ddotp}} % doesn't work
}
\begin{document}
$\dots, \ldots, \cdots, \vdots, \ddots$
\end{document}
Found an example of where my solution for \cdots didn't work. It's when \dots is used instead of \cdots explicitly to centrally place dots:
\documentclass{article}
\usepackage{unicode-math}
\AtBeginDocument{
\renewcommand\mathellipsis{\mathinner{\ldotp\ldotp\ldotp}}
\renewcommand\cdots{\mathinner{\cdotp\cdotp\cdotp}} % sometimes works
}
\begin{document}
$x+\dots+ y$ % Do not get spaced dots
$x+\cdots+ y$ % Do get spaced dots
\end{document}