For the following equation, an error shows up
TeX capacity exceeded, sorry [semantic nest size=500]. ...(\vm\right) & a_2\left(\vm\right) & \cdots &
while removing some elements from the array eliminates the error.
\RequirePackage{luatex85}
\documentclass{scrbook}
\usepackage[tuenc,no-math]{fontspec}
\usepackage{unicode-math,mathtools}
\newcommand\vm{\symup{x}}
\begin{document}
\begin{equation}
\mathbf{a\left(x\right)}^T=
\{
\begin{array}{c c c c}
a_1\left(\vm\right) & a_2\left(\vm\right) & \cdots & a_m\left(\vm\right)
\end{array}%
\}
\end{equation}
\end{document}
For the same equation, if unicode-math
is removed, and the code changes accordingly, the error doesn't show up.
\RequirePackage{luatex85}
\documentclass{scrbook}
\usepackage[tuenc,no-math]{fontspec}
\usepackage{mathtools}
\newcommand\vm{\mathrm{x}}
\begin{document}
\begin{equation}
\mathbf{a\left(x\right)}^T=
\{
\begin{array}{c c c c}
a_1\left(\vm\right) & a_2\left(\vm\right) & \cdots & a_m\left(\vm\right)
\end{array}%
\}
\end{equation}
\end{document}