Ultimately it does depend on the player though, I was just generalising after all. I do have a feeling the Phoenix's style of football might suit a Spanish player provided we keep the ball on the deck and play an incisive passing style (build the team around Ferrante and Daniel) rather than long ball. It's pretty obvious that Morientes was never going to succede under a long ball style (ok, Liverpool aren't quite Bolton but still...) because it's just not what he's used to and he didn't have the mental toughness to adapt.
Maybe you can also make a case for age playing a factor. Morientes wasn't a kid by the time he moved to Liverpool and was probably reluctant to even try and learn the new style, whereas the likes of Cesc and Arteta are younger and have the hunger to play regardless of how tough it is. Younger players can be molded as well, but of course there is the maturity factor (homesickness, new language, newfound independence and responsibilities etc) that comes into play with signing kids.