Whilst on the board with both feet attached in the bindings, any twisting force occuring will twist the whole body as one - both legs and trunk must therefore move in the same direction. Snowboards tend to be shorter than skis which reduces the lever arm of force produced by any twist. Although knee injuries are rare in boarders, it is on lifts that they occur. Well, the lead leg is still attached to the board whereas the trailing leg is not. Head injuries are usually the consequence of a fall.
Most head injuries are thankfully minor in nature.
However, with increasing force of impact the risks increase and can include death or long term disability. Nine were serious in nature - three required brain surgery and three were left with long term brain damage. There is little doubt that wearing a helmet makes sense. They are, however, still viewed as uncool and not trendy. If you are buying a helmet try and get one that meets either the US standard RS 98in or the European standard EN1077.