I know aluminum conducts heat away 100 times faster than air, but as with everything, common sense is all that is required. If someone puts an aluminum whistle which is at -30 degrees Celsius in their mouth and it sticks to the lips then surely that is probably the best place for whistle to end up anyway (visions of Dumb and Dumber come to mind) as the idiot using the whistle shouldn't probably have a PSK anyway. He might eat the Esbit tablet, burn his fingers with the hot part of his lighter, ask where the safety razor is - ask why does the shaving mirror have a hole in the middle and god forbid wonders what he should do with their condom in a survival situation especially at -30 degrees.

Although the whistles you mention are SOLAS and US Coast Guard approved and the aluminum Lifeventure model isn't, this is because the models you have mentioned will float. They are not any louder or anymore functional. The main disadvantages are that they are relatively large at take up a lot of space in a compact PSK. There is always the problem that a whistle may be left out of the PSK because of the size constraints of the PSK container. Traditionally, whistles have not been a part of a PSK. I would rather have one, albeit aluminum which is roughly 1/3 the size, rather than none at all in my PSK.

Update (edited) - The whistle is also referred to as a Lifesystems Mountain Whistle. - Recommended by the Mountain Leader Training Board (UK) as an essential item to be carried in the hills.