Not tried an Alios, use an Ecrin Roc and have been happy with it. Not the last word in svelte but the problem is most of what you're doing is looking to stop falling objects and the more space bwteen the top of the lid and the crown of your head the more space there is not to get a rock embedded in your skull...
In a climbing/caving helmet where that's the main game you have air over the shock cradle, with the cradle distributing the load across your head as the shell itself deforms into the spare space. With a polystyrene liner that impedes air flow compared to spare space and you've got a major component of the shock absorbtion system prone to brittle failure (if you see a post-accident cycle helmet that's cracked it's entirely possible it hasn't actually done its job, suffering brittle failure which only absorbs a fraction of the energy of deforming the polysytrene).
While on the one hand it is nice to have a one size fits most, fact is that it'll be a compromise and any compromise won't be as good as the purpose built alternatives, so unless you can't afford a selection or you want to use them all on one trip there is a clear downside. So less good (I guess) at falling rocks than an Ecrin, less well ventilated than a roadie's helmet, less enclosing than a paddlers whitewater helmet (so easier to getthe side of your face ripped on rocks as you traverse rapids upsid-down).
Pete.
