I'm more of a rtw 1 sucker myself, but since ME2 is is built on the same engine I'd assume they kept the penalty system in. Note that not all terrain/weather bonuses/penalties are listed in the unit info cards, some of them can only be found by nosing around in the game files. You did have heat penalties in deserts for non-native troops in rtw 1. If I recall, they acted like a slight debuff to attack/defense and faster accumulation of fatigue.