I read a Helmet rating review article a while back.
The summary was, Snell went more strict about rating approvals, than just DOT compliance. Partly to differentiate from simple DOT compliance ( a value add market/sale offering). However, it also progressed/evolved to become more about helmet survival (it staying intact) than it did about skull and brain bruising protection.
Personally, I think it's perfectly fine to destroy a helmet in a crash in order to protect/save brain/skull. The issue revolved around the space between the brain and skull which allowed the brain to stay in motion when the skull stopped suddenly. The true goal should be for the helmet to decelerate the skull speed to minimize the brain bashing into it.
While the snell ratings meant better survival of the Helmet, the same could not be said of brain survival in a crash. Crash energy is dispersed when the helmet shell breaks up in a force absorbing action. Forces dispersed outside of the skull and brain, are a good thing.
I think snell downplayed this latter attribute as marketing a stronger helmet that stayed intact in high speed crashes was an easier sell, testing-wise, than making a case for brain decelleration/survival.
Another fact to be aware of, is that the brain shrinks a bit inside the skull as we age. This makes it harder for any helmet to protect the brain from rapid deceleration forces. We sure aren't made to last forever.
My helmet experience has ranged from none (foolish youth), to open face, to full. The last 30 years have mostly been with Shoeis, and always had the issue of pressure on the forehead. I just recent bought an Arai made for more elongated skulls like mine. Felt good, if a bit tight, in the store. I don't have much riding experience with it yet though.
Cheers,