They are essentially opaque, i.e. saturated, in the STEM and so yield little information. For filaments or two- dimensional arrays, the actual size is not a problem. It is only the mass density that is of concern, so they are fine if they are spread out enough and not too dense. The physical purity of the specimens is very important for STEM preparations. Essentially everything is visible in the STEM, and impurities may well not wash off the grid.
Most biologists have some assay for the biological activity of their specimens.
However, many preparations contain biologically inert but physically visible material that may make it difficult to obtain good STEM data. Particulates from columns or gels are a problem. Additives for activity such as BSA, PEG, or trypsin are a problem. Precipitations can concentrate the specimen, but also enrich contaminants. At times, the particles of interest can be distinguished from contaminants.