I know from experience that the habit of avoiding controversy can in the end cause far more trouble that it avoids, because strong feelings, unexpressed, don't die but build up.
About the because-clause, which part does it modify?
(1)I know from experience that the habit of avoiding controversy can in the end cause far more trouble that it avoids, because strong feelings, unexpressed, don't die but build up.
(i.e. the sentence in that-clause only)
(2)I know from experience that the habit of avoiding controversy can in the end cause far more trouble that it avoids, because strong feelings, unexpressed, don't die but build up.
(i.e. the entire sentence in front)