Compiling this example:
\documentclass{scrartcl}
\usepackage{fontspec}
\usepackage{mathtools}
\usepackage{unicode-math}
\setmathfont{latinmodern-math.otf}
\begin{document}
\begin{equation}
\int\limits_a^b x² \,\symup{d}x
\end{equation}
\end{document}
with lualatex looks a lot uglier than compiling with xelatex.
The spacing of the \limits is just way out of perfect:
lualatex result:
xelatex result:
