I don't agree. I played my first game on immortal (tradition), and phew, in the end it was very tough to win a science victory, due to the fact that my cities became so unhappy (Maybe I grew them too big?). Late game was a constant battle against unhappiness. i Had to build wealth in 3 out of 5 cities for the last 100 turns, it seems - I don't know - it just felt like forever. To have more unhappiness to deal with, in late game, as others has already complained about, would for sure had ruined the game. Unhappiness and money shortage was a real problem. I even had to sell of 2 spaceship factories before they could help finish their job! And very much too late I realized I should have sold off those "food producing horse buildings" much earlier...I think the happiness is a little too easy now. I hardly need to manage it at all.
I was at least 7 techs behind all game long (And I hadn't even started my third policy tree, when Spain unlocked ideology!), and in late game I came around and was at most 7 techs a head. But due to unhappiness and money shortage. One of the AI's got her final tech for her last spaceship part, the turn before I launched my spaceship! So please no more unhappiness, because I already think it's too hard!

Is there a guide explaining the happiness system somewhere to be found, explaining the relationship between research & happiness, and city size etc.?