I agree with the above post. We have to define success along some lines, at least, before we can prove that the novel's ending was unsuccessful. 

I've heard a variety of arguments about the ending of this novel with many of those arguments suggesting that Twain faced a challenge in bringing this novel to a close and failed to overcome that challenge. 

However, there are many open-ended narratives out there that are satisfying, so I think we have to consider why people think this book is a particular failure in its closure. Maybe Twain brings something into question - a specific conflict - and fails to resolve that conflict. This type of analysis is one way to nail down a definitive judgement.

Twain's ending may be considered "unsuccessful" as it does not resolve anything.  In fact, there are two interpretations of the ending: Huck avoids responsibility for his decision, his "conscience suffers defeat" as Twain himself writes, or Huck flees from what he recognizes as a hyprocitical society in which he does not want its "civilization."

Of his novel Twain wrote,

a sound heart is a surer guide than an ill-trained conscience", and goes on to describe the novel as "...a book of mine where a sound heart and a deformed conscience come into collision and conscience suffers defeat.

Perhaps, even Twain felt that the resolution was unsuccessful.

How do you define "successful"? By the end of the novel, Tom and Huck were back home, Jim was free and reunited with his family - that sounds pretty much like a happy ending to me, unless you are in sympathy with Huck's reluctance to becoming too civilized.

Tom's actions toward Jim were cruel, although not surprising for his personality. That doesn't necessarily make the conclusion unsuccessful; it just makes it harder to read and stomach.

This is an interesting question. Ernest Hemingway said that everything after losing the slave Jim was cheating on Mark Twain's part. Even with that said, he considered it to be America's greatest novel.

In some ways, I do think the ending is a cop out.  But I am torn because I consider Tom Sawyer's return another level of satire against slavery as an institution.  Tom's elaborate plan to free Jim, even though he knew he was free, kind of indicates the polar opposites the two are.  For Huck, keeping Jim as a slave is immoral.  For Tom, setting him free is immoral.