Depends on how extreme. I'd rather ride in +40ºC than -40ºC. I've done a little riding around the city in -40 weather, and the bike is simply much harder to pedal at those temperatures - bearing lube is too thick - and the thickest gloves imaginable still seep heat from your fingers. I wouldn't dream of touring in that.

But I might also consider -10ºC to be "too cold," but would still be more pleasant than anything much over 30ºC. You can dress for the cold, and particularly when it's only a little cold, the exercise counteracts the effect. With hot, there's a point at which it's simply impossible to get more comfortable by adjusting clothing.