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.