High contrast typefaces are the ones that have a drastic difference between strokes in a single letter shape (see image 1). As opposed to low contrast typefaces which usually have monolinear stroke size. However, this doesn't mean typefaces that appear to be monolinear have uniform thickness across the letter shape. All letter shapes are optically corrected to appear of the same size (see image 2).
The higher the contrast that means more pixels required to render the details properly. Which in turn requires the text either to be large in size or the resolution of the headset to be considerably higher.
In high-contrast typefaces, the pixel loss tends to happen in the areas of thin strokes and the user is unable to perceive these areas making it harder to read in small sizes.
Moderate contrast still works better in low-resolution.
Serif typefaces with moderate contrast still perform better than high contrast. As they appear to be slabs serifs even when they lose brackets in serifs.