Cold/Heat effects for nations that prefer one?

[Vicious Love]

Units don't have a preferred temperature. Heat or Cold 3 will cause fatigue/encumbrance/whatever to units without the appropriate resistance(Fire for heat, frost for cold), and that's it. No other direct effect of temperature on units, save seasonal spirits and other special cases.

[Tuna]

Well, heat/cold has a difference for cold-blooded creatures. Apart from that, no difference.