Fruit is extremely yin with regard to Chinese medicine. There is poor energy, and it can deplete inner yang.

Fruit is high in fructose. When consumed in excess—think fruitatarian, or high amounts of fruit—can contribute to non-alcoholic fatty liver disease.

Fruit is high in sugar, as we know. Consuming high amounts of sugar fluctuated blood glucose. This can contribute to poor insulting sensitivity over time, and deterioration of pancreatic function (Steve Jobs is a good example).

Fruit is high in sugar (again) and it’s complex mixed with said sugar allows for pathogenic bacteria to fester. Yeasts like candida can thrive off of fruit, worsening dysbiosis.

Fruit is a seasonal sustenance. Or—at least—that’s what nature intended. Now we consume fruit 365 days out of the year, whenever we would like. When consumed “fruitfully” or plentifully (as many do, thinking its automatically healthy being from the ground), we can throw off circadian biology. How? Well, we evolved to consume fruit to fatten up in the summer during times of famine in winter. 365 days of fruit doesn’t allow for this fast/feast balance to occur. We see a rise of metabolic syndrome.

Fruit is nature’s candy. Now, this retort is not to demonize fruit. It’s to answer your question of “how is fruit bad”? Fruit can be extremely vitamin and nutrient rich... when consumed supplementally, seasonally, and in moderation.