Using mhchem with gfsneohellenic gives a weird looking arrow

By default, mhchem's arrows are composed by characters from current math font. This requires the corresponding glyphs being able to pieced together, and gfsneohellenicot seems to be not capable to do this. Extendable math arrows like xleftarrow and xrightarrow will give similar broken output. (In fact, mhchem re-used xrightarrow's code, from the amsmathpackage.)

For mhchem, you can config it to draw the arrow using tikz. For example,

documentclass{article}

usepackage{gfsneohellenicot}
usepackage[version=4, arrows=pgf{To[length=3pt]}{.15ex}]{mhchem}

begin{document}

begin{equation*}
  ce{H2 + 1/2O2 -> H2O}
end{equation*}

end{document}

gives

• The full document of package option pgf={<arrow tip>}{<line width>} can be found in documentation of mhchem, subsection "Arrows" (page 18 for v4.08).
• You can also use things like ce{->} to substitute broken extendable arrow commands, as well.