These "forbidden" substances are usually too hard for a medic to treat and kill slowly. In this case, the thing stick and can burn till the bones, and even after the initial impact the remnants can still ignite. It is just a mess in the body.

Upon contact, white phosphorus can burn people, thermally and chemically, down to the bone as it is highly soluble in fat and therefore in human flesh. White phosphorus fragments can exacerbate wounds even after treatment and can enter the bloodstream and cause multiple organ failure. Already dressed wounds can reignite when dressings are removed and the wounds are re-exposed to oxygen. Even relatively minor burns are often fatal. For survivors, extensive scarring tightens muscle tissue and creates physical disabilities. The trauma of the attack, the painful treatment that follows, and appearance-changing scars lead to psychological harm and social exclusion.

Just to reiterate, cuz it really sounds like some sci-fi alien shit:

Already dressed wounds can reignite when dressings are removed and the wounds are re-exposed to oxygen.

Think Napalm's meaner acidic cousin

What makes this worse than other things?

I think it's good to read the article, they explain a lot of it and how Israel is refusing to comply with its ban as a weapon.

Sorry, you were right, it doesn't explain too much in the article, but this from HRW explains it in much better details: https://www.hrw.org/news/2023/10/12/questions-and-answers-israels-use-white-phosphorus-gaza-and-lebanon

I caused confusion, it was my bad.

Chemical rounds meant to cause injury are banned in the Geneva convention. It's not a separate treaty.

If it's being used as a smokescreen, a very common use, there is nothing at all bad about it.

Edit: People seem to think that it's common use as a smoke screen is a problem. What, specifically, is the issue when it is being used as a smokescreen?