Your concern is 'length differences'. On average over the life time all these 'differences' even out. Argentina will have long days of fasting in a decade or so. At that time sweden will have 8-9 hours of fasting day. I would say on average every muslim will have same amount of fasting over his/her life time.
As for how much rewards one gets is unknown to any of us. Allah is just so trust Him fair reward.
P.S.1 Next people living in hot climate around equator should complain why fasting is difficult for them as compared to people living closer to north/south pole.
P.S.2 I for one satisfied that in my life I have experienced shortest, longest, and moderate length fasting days all with various degrees of toughness due to heat, cold, dryness etc while living in Pakistan and Europe.