While I haven't found particularly good advice on scanning negatives, I have found that the film thickness can be sufficient to place the "picture" a significant enough distance from the scanning surface to give some loss of sharpness.
Most of the negatives I've tried scanning were very old B/W, but I'd expect even more difficulty with color, since there are usually three separate layers of images, with filter layers between them, in most kinds of color film. The multiple layers should be thin enough to cause little dispersion with some scanners, but seem to be sufficent to "fuzz it up" with others.
Some scanners essentially "wave a light bulb" past the image they're trying to pick up, while others do a fairly good job of "focusing" decently colimated light that doesn't disperse too much to pick up a decently accurate reflected image for the depth of the film. I've seen a visible degradation if the negative isn't carefully placed "emulsion side down" in one of my "photo scanners" but little of the same effect on one other. A "quicky" method of seeing if a scanner has some depth of field might be just to scan a "wrinkled" sheet of paper and see if it blurs where the paper is "off the plate."
An accurately colimated illumination source would seem to be a requirement for a "transmission" scanner designed specifically for negatives, but I haven't had one of those to play with. It would be an expected feature in a "negative scanner," although finding specs sufficiently detailed to include that much detail likely would be difficult.
One of my multipurpose scanners had a "transparencies mode" setting, but I hadn't bothered trying to figure out if it worked; and it was an HP so I didn't have occasion to try it out before the scan bar crapped out - 13 months out of the box. (But the printer still works just fine.)