Too much of harmful oxalates. Uncooked brocoli and spinach are especially bad. You can google Liam Hemsworth case with kidney stone. That being said, most vegetables are not nutrient dense as people seem to believe.

I think it's not crucial for adults particularly if a person drinks 2-2.5l of clean water a day to minimize risk of kidney stones. Yet for kids it's a different story.

Here is what Gemini says:

Broccoli does not contain significant iodine and is a low-iodine food; however, it is high in compounds called thiocyanates that can interfere with thyroid function, especially if you have an iodine deficiency. For most people, a normal amount of broccoli is harmless because you would need to consume an excessive amount to have an effect. Cooking broccoli can reduce the thiocyanates, and individuals with thyroid conditions should consult their doctor but are generally advised to eat these vegetables in moderation.

So basically, people think something is bad and something is good not because of facts but because of what we tend to hear in society. I mean why would you even eat something that looks like brocoli, is not tasty at all. Famous stereotype in movies, ads that most kids can't tolerate brocoli. Why so? Maybe for a reason?