Two rather big flaws
This was an enjoyable revenge western with some gorgeous photography (where else do you get location-shooting in the desert, a swamp, and the mountains?) However, it has two major shortcomings that can't really be ignored:
1. Steve McQueen. I like McQueen as much as the next person, but as a young half-breed he's completely unconvincing. Not only does he strain credulity, but his performance leaves a lot to be desired. He covers the "novice, out of his element" aspect well enough, but as a guy consumed by vengeance, he treats his "quest" like a "lark".
2. Based on his time spent with the preacher, we're supposed to believe that he has "turned the other" cheek by not killing Malden. That's all fine and dandy, but it came at the expense of Cord life. There's no atonement as he rides off, presumably into the sunset, in blithe spirits while Cord lay dying on the other side of the ridge. He assumed culpability for what happened to Pilar, but not Cord. That's a pretty grievous ending, in my opinion, because he does not learn from his experience with Pilar but we're supposed to accept his spiritual "transformation".
"...if that was off, I'd be whoopin' your ass up and down this street." ~ an irate Tarantino