Whether you like it or not, for someone who lived most of their life in a climate where it is warm for 9 months a year and the associated lifestyle, it can be important. It's not *the* most important factor; if it was he would move back to Portugal...and he's not going to move back to Portugal because their clubs aren't competitive and can't afford wages.
However, if you can get the same money as at City, playing in the CL, for one of the most historic clubs (certainly seen as the biggest & most important from most on the Iberian peninsula), in a beautiful and warm city, and be nearer to your family...then that is going to have some appeal.
It's not a black and white decision, there will be plenty making him want to stay (trophies, chance to win CL, fans who love him, teammates, Pep)
but pretending that some players who grew up in a Mediterranean climate aren't at all arsed that it is miserable in UK except for a random 5 weeks a year is a bit odd. Add the other factors (family, new challenge, boyhood dream) and its a tough choice.