Chang has made 4 slam finals. Ferrer one. Chang has won 7 Masters, Ferrer only 1. The two are not in the same league. You also cant prove Chang wouldnt have reached any slam finals or won any Masters in this era. Ferrer has lost slam semifinals to a very subpar Murray (2011 Australian), a very young Djokovic (2007 U.S Open), and lost in slams in his best surfaces numerous times to lower ranked opponents, and so on. Tsonga, Soderling, Robredo, Berdych, and others have all won their first Masters before he could. Chang has atleast beaten the likes of Agassi, Courier, Edberg, and Lendl in slams. Ferrers only ever big wins in a slam are over Nadal on hard courts, that is it, and no beating Murray at RG is definitely not a big win, sorry.
I do agree the very top players with a big games were confident vs Chang, just as they were with Ferrer, but Chang was still harder work and a more dangerous foe than Ferrer was.