Math/equation keys in pgfkeys and tikz picture

Question

I am using pgfkeys as variables of a newcommand for TikZ pictures. I can transfer numerical values and simple math correctly. However, some math expressions are not transferred to the command. A MWE is provided below.

Why an equality expression and the \unitfrac are not working?

\documentclass[]{standalone}
\usepackage{units}
\usepackage{siunitx}
\usepackage{tikz}

\pgfkeys{/mycom/.is family, /mycom,
default/.style={text = $?$},
text/.estore in = \mytext}

\newcommand{\mycom}[1][]{
\pgfkeys{/mycom, default, #1}
%some tikx command to display the math
\draw (0,0) node{\mytext};
}

\begin{document}

\begin{tikzpicture}

%\mycom[text = aa]                         % Does work
%\mycom[text = $5x$]                       % Does work
%\mycom[text = $5x+6$]                     % Does work
%\mycom[text = $5x=6$]                     % Does not work
%\mycom[text = $\unitfrac[10]{kN}{m}$]     % Does not work
%\mycom[text = $\si{\kilo\newton/\meter}$] % Does work

\end{tikzpicture}

\end{document}

Answer 1

The = needs to be protected to stop PGF trying to parse it. For the second case, you need not to expand it so soon. If you use .store rather than .estore, this all works just fine.

The counter is only for testing because the uncommented code typeset all nodes in the same place.

\documentclass[border=10pt]{standalone}
\usepackage{units}
\usepackage{siunitx}
\usepackage{tikz}
\pgfkeys{/mycom/.is family, /mycom,
  default/.style={text = $?$},
  text/.store in = \mytext}
\newcounter{foo}
\newcommand{\mycom}[1][]{%
  \pgfkeys{/mycom, default, #1}%
  %some tikx command to display the math
  \draw (\thefoo,0) node {\mytext};%
  \stepcounter{foo}%
}

\begin{document}
\begin{tikzpicture}
  \mycom[text = aa]                         % Does work
  \mycom[text = $5x$]                       % Does work
  \mycom[text = $5x+6$]                     % Does work
  \mycom[text = {$5x=6$}]                     % Does not work
  \mycom[text = {$\unitfrac[10]{kN}{m}$}]     % Does not work
  \mycom[text = $\si{\kilo\newton/\meter}$] % Does work
\end{tikzpicture}
\end{document}