The critics mostly pan "American Masters" because it glosses over the controversial and the un-appealing aspects of the lives of the featured artists. In many cases this is sbsolutely true, though there is almost always an effort to feature the artist speaking of their careers - a trade-off that makes it worth it in many aspects. A critic or an academic can often have a valid perspective, but the words of the people who did it have a lot of historical value in their own right.
"American Masters" has moved toward a more popular and uniform format in the last five years, something the viewer can easily notice. Well-known musicians, actors, comedians, and artists have largely replaced the more variable mix of the early seasons (sculptors, architects, choreographers, etc.). This may have stabilized the longevity of the program (during the lean years of the early 90s, WNET produced or acquired very few new installments), but I kind of miss the variety.
But, being as objective as possible, I would say this program is actually more focused than ever. Though it may have given up a rawness of following people through their working days, it at least still insists on the artists "in their own words" whenever it can.