I have been trying to fake small caps which are not available in Myriad Pro. So I used the Scale and FakeBold options. Scaling is allright, but as you all know the glyphs slim out. To work against that I tried to "fatten" them somewhat.
The code below does not "fatten" the scaled characters when using lualatex (latest TeX-Live). Using xelatex (also latest TeX-Live) works fine.
I think its a bug.
\documentclass[11pt,a4paper]{article}
\usepackage{fontspec}
\setmainfont{TeX Gyre Heros}
\begin{document}
ABC {\fontspec[FakeBold=5,Scale=0.72]{TeX Gyre Heros} ABC} ABC
\end{document}
For demonstration the code above uses another fontwich actually supports small caps.