The concern is due to the presence of methanol (wood alcohol), an optic nerve poison, which can be present in small amounts when fermenting grains or fruits high in pectin. This methanol comes off first from the still, so it is easily segregated and discarded.
Methanol is an especially nasty type of alcohol because the body tries to break it down the same way it metabolizes, or breaks down, ethanol, the type of alcohol in beer, wine and other drinks. Metabolizing ethanol produces chemicals less toxic to the body than alcohol. Unfortunately, if the same chemical action is performed on methanol the result is formic acid, lactic acid and formaldehyde.
Formaldehyde attacks nerve cells, especially the optic nerve and can damage the liver and kidneys. Formic acid and lactic acid also attack the kidneys and liver. Most people who have drunk methanol die of severe and sudden kidney and liver failure.
Methanol is a by-product of fermentation; more methanol is produced in fruit fermentation than in grains. Brewers do not remove the methanol in beer and wine because methanol is not especially toxic at low concentrations. You are looking at between 0.4%-1% methanol in wines and brandies and smaller amounts in beers. Distillers remove almost all the methanol in most cases.
Chronic methanol drinking will cause optical damage. The stories of moonshine causing blindness comes from U.S. prohibition times where some bootleggers used to cut moonshine with methylated spirits to increase profit.
So the moral of the story is twofold: 1) if you don’t know what the hell you’re doing and you’re attempting to make homemade moonshine, consult some credible research sources first so you know what you’re drinking is safe, and 2) the chances of you going blind from drinking any “legal” moonshine are nil.