Ebert was a bit notorious for some of his reviews, and I had my WTF moments with him as well. Usually he stood by his opinions, though there were a few occasions when he admitted he was wrong and revised his initial impressions. He described his own criticism as emotion-driven and visceral rather than analytic and cerebral. He said that ideally you have to keep an open mind and approach each film afresh, not repeating the same judgements.
I do not recall “The American” being that bad, but it came out enough years ago that I really need to see it again before making further comments about it.
By the way, I may have misrepresented Ebert’s “star” system. While he did repeatedly explain that the stars are relative, and not absolute, in “You give out too many stars” he had this to say: