I am in a unique position to confirm that they are a load of bunk. I have solar urticaria and develop hives in response to UV exposure, directly proportional to how much UV is getting through. I’ve developed hives in minutes while the UV index was supposedly only 4 and gone for relatively too long without erupting in hives the next day even when the UV index was supposedly 10.

i hate to be that person that quotes chatgpt, but this seems VERY relevant to your complaint:

“Solar urticaria is a rare condition where the skin reacts to specific wavelengths of light rather than the overall UV intensity. The UV index is a general measure of the total amount of erythema-causing UV radiation (mainly UVB) that can cause sunburn in the average person.

But in solar urticaria, the trigger might be UVA, visible light, or even a narrow band of wavelengths — and the UV index doesn’t capture that nuance.

So it’s not that the forecast is wrong — just that the UV index isn’t designed to reflect the sensitivity profile of solar urticaria.”

In other words, you’re (literally) a special case. :)