I do not agree with this assessment.
In my current game as Austria, I've fought 4 wars in Italy before 1480. The first two yielded me two of the papal state provinces, and 3 of the continental ones from Venice. Cored them, and added them to the Empire. The third one knocked out Venice and Istria, and handed the remaining continental one to Milan (one of my allies). And then, in the 4th war, I took out the remaining 3 papal state provinces, cored them, and added them to the empire.
At no point was the AE anywhere near dangerous levels. Nor was the overextention. Coring of these provinces took about 33 months each, which I don't find exceptionally long. The malus of AE in the relationships was gone after only a few years with each of the european nations (princes and electors alike), and most relations stayed well in the +100 to +200 range. So no coalition against me. Most of the time, I had support from all electors except Brandenburg and Bohemia. At a minimum, I had the support of at least 3 of the electors at all times during this period.
Although, I should mention I made sure I had Milan, France and Hungary as allies before I kicked of the first or the wars. These three turn out to be exceptionally loyal allies, who not only helped me putting things in order in Italy, but also come to the rescue whenever I, as Emperor, need to teach a Prince (sometimes even an elector) that it's not nice to conquer a neighbor. Naturally, I've helped France fight it's wars too, although sometimes these came at extremely inconvenient times.