I am making a map that includes all kinds of vector layers; the map looks perfect in QGIS 2.12 Lyon as well as in map composer. None of the layers have scale-dependent visibility.
I have the following issues when exporting as image (regardless the output format):
A4 format: If I set the resolution to 200 dpi I get an image with all the layers displayed. If I set the resolution to 250 dpi I don't get some vector layers in the output image (such as 25 meters contour lines and some land coverage). When I save with 300 dpi resolution I only get a basic map excluding polygons and all the contour lines.
A3 format and more: similar to the above, but already at 200 dpi I only get few layers printed in the output raster file.
The same project printed in QGIS 2.8 doesn't have the same issues and all the vector layers are exported properly.