An extremely good rule of thumb in eating is that "too much and too little of something is bad for you", and this holds surprisingly well for the vast majority of nutrients. For the vast majority of nutrients the body needs a good balance of it. Too much or too little of it is not good.
However, for some reason many people do not understand this, and instead tend to go to (often unhealthy) extremes.
The (rather primitive) rationale is "this nutrient is healthy, thus eat it as much as possible" and "this nutrient is unhealthy, thus avoid it completely". Actual example: Many people eat vitamin supplements like they are candy. In fact, some people have suggested extreme overdoses of eg. certain vitamins (most typically vitamin C) as being a good thing. (There was an actual, thankfully relatively short-lived, movement that advocated eating enormous amounts of vitamin C.)
Like with most nutrients, vitamins are a good example where a good balance is crucial: If you get too little you will get vitamin deficiency symptoms. However, too much of it can be poisonous or otherwise hazardous. (With some vitamins the overdose threshold is much lower than others, eg. vitamin A is one of with the lowest thresholds, and vitamin A poisoning is a very serious condition. However, all vitamins have overdose thresholds.)
Since there's a general notion that "processed food" is unhealthy and "natural food" is healthy, this, of course, also causes extremism. One of the most extreme forms of this is the so-called "raw food" movement, which advocates eating solely uncooked vegan food.
The "raw food" movement is not deceptive and deluded only in that they completely ignore actual scientific studies regarding cooked vs. uncooked food, but in fact they go so far as to outright fabricate hazards of cooked food that do not exist, fabricate benefits of a raw-food-only diet that are false, and try to use scare tactics to "convert" people.
While a raw food vegan diet is not in all cases harmful, it can still be lacking in some nutrients and cause all kinds of deficiency symptoms, both in the short run and the long run. (Many people who switch to a raw food vegan diet get short-term symptoms like diarrhea and other symptoms. Naturally the raw food advocates will just reassure such a person with things like "it's just your body getting rid of all the toxins" without even a shred of actual scientific evidence of this.)
The advocates do not understand what cooking does to food, and will dismiss and laugh at scientific studies that indicate benefits of cooking. Cooking does not only sterilize food rather effectively, but it can also increase its nutritional value in some cases. The advocates do not understand (nor want to understand) how the latter is possible, and will just laugh at the claim.
Heat causes chemical processes in food. Many of these processes are destructive. That term, however, is a bit misleading because the intuition is that "destructive" always means "bad", ie. that the food becomes worse by cooking. In fact, "destructive" simply means that bigger molecules get broken into smaller ones. In some cases this does mean that some beneficial nutrients are broken into non-beneficial components (or at least components that are not as beneficial.) However, in other cases the parts are actually more beneficial for the body because it makes it easier for the digestive system to process them. (In other words, the digestive system has hard time breaking those original bigger molecules itself, and cooking breaks them into parts that are more easily digested.) In other words, cooking increases the nutritional value of food in many cases.
The nutritional value of most food, cooked and uncooked, is listed out there, so you can look it up.
For a third example, it has for a long time been thought that antioxidants are beneficial because they prevent cancer by destroying free radicals. And this is indeed so: If you have too many free radicals in your body, antioxidants will decrease the risk of cancer caused by them. However, like with most other things, also this is something that the body requires a good balance of.
Recent studies have shown that free radicals not only can cause cancer (when there's an excess of them), but rather ironically, they also prevent cancer (when their amount is just right.) In other words, rather ironically, too many antioxidants, and thus too few free radicals, can increase the risk of cancer. In other words, people who pop antioxidants like candy are actually making themselves more prone to cancer in the future.
Like with most things, the amount of free radicals in your body needs to be balanced. Having too much of them is a bad thing. Getting completely rid of them is a bad thing.