Every kind of food processing has trade offs including cooking which destroys vitamin C among other things.

White rice is missing thiamine (B1) which historically was killing off significant numbers in Asia as it became popular. There’s plenty of historic examples with similar issues. Now days foods are often fortified with various things to cover the most common and severe issues, but that’s probably not enough on its own.

In terms of highly processed foods you run into various additives. There’s a long list of minimally studied additives/dyes/preservatives/etc being used only for them to later be recalled and replaced with some other poorly studied substances. Ignorance may be a legal defense for Nabisco adding X to their products, but it’s not going to protect your health.

So, the advice is basically to have the absolute minimum amount of processing steps which is a surprisingly useful heuristic alongside having a varied diet including lots of different plants.