I think that you run into the same kinds of problems when you try to compare careers in just about any field. There are consistently good performers, consistently bad performers, and a bunch of people with a mix of good, bad, great, and awful performances.
If Woody Allen had retired after "Annie Hall," I think that he would be almost universally regarded as one of the greatest comedic film directors, even though his directing career was still relatively brief. But after "Annie Hall" (which I consider to be one of the greatest comedies ever), where could he go? He didn't want to return to pure comedies like "Bananas" and "Sleeper," which involved characters about whom we weren't supposed to care much. He tried to imitate Bergman with the very dark "Interiors," which might have worked better if he hadn't been so strongly associated with comedy by that time. I thought that "Manhattan" was a successful return to comedy, but with a certain sadness and wistfulness. (I remember a review that described "Manhattan" as "'Annie Hall' in the shadows, 'Interiors' in the sunlight.") After that, it seems as though he's made a long series of films that have ranged from pretty good to excellent. As you suggest, though, they tend to look and feel very similar.
I avoided the term "overrated" in discussing Spielberg, and I've never even seen "ET." Regarding "Schindler's List," I remember one scene that really tore at my heart (as did a couple of scenes in "Saving Private Ryan"), but I've always felt that Spielberg's movies seem to pull back when they get too close to the edge, as if there's some sort of emotional set point that he always has to return to before the final credits roll.