Matoran, maybe... while they do have a higher resistance to extreme temperatures, I imagine there's a limit to just how long that resistance can hold out before it starts affecting their bodies. After all, resistance doesn't always mean they'd be completely impervious to it, much like how we keep going in lower or higher temperatures for a while before it starts taking a toll our bodies. As for what Jaller said, I'd say that's because Toa are more likely to be able to do so, what with them having full control over their powers and they likely get a boost to the resistance to their element when they transform. Plus if they have a Hau they could always just shield themselves for added protection. :v