thanks for the effort. i can’t say i understand anything about fonts if i
judge from what the results of above suggestions look like:

    \usemodule[simplefonts]

    \definesimplefonttypeface[mainface][Cambria]
    \definesimplefonttypeface[mainface][Ubuntu] [style=sans]
    \definesimplefonttypeface[mainface][cambria][style=math]

    \setupbodyfont[mainface]

Well: header math is in unbold cambria, body text in courier or something
like this, and the math font features are not used (tiny parentheses around
matrices, fraction denominators and numerators are shrunk instead of using
small glyphs)

    \setfalse\completeboldmathstrategy

    \usemodule[simplefonts]
    \setmainfont[Cambria]
    \setmathfont[Cambria]
    \setsansfont[Ubuntu]

too slim parentheses, but this is to be expected :D

    \definetypeface[mainface][rm][specserif][Cambria]
    \definetypeface[mainface][ss][specsans] [Ubuntu]
    \definetypeface[mainface][mm][math]     [cambria]

    \setupbodyfont[mainface]

exactly the same as above. slim parentheses in header math, rest working.
cambria in the last line only works when lowercase, but is case-insensitive
in the rm and ss lines. does this solution disable the
completeboldmathstrategy automatically?

thanks for the fixes,
philipp