The perhaps unsavory union-organizing methods employed by the United Farm Workers were fairly tame by most historical standards. Union and anti-union violence - threats, beatings, bombings, sabotage and murder - especially against labor organizers, strikebreakers and scabs, is well-documented in the history books. I think it's fair to say that the UFW's methods were less violent and corrupt than that of the Teamsters or Mineworkers and their Union Thugs on one side, or the Owners and Bosses with their frame-jobs and bullying by Pinkertons and Goon Squads on the other.
There are neccessarily strong-arm methods to organizing a mass of disadvantaged, frightened workers in the face of management's union-busting activities. It's not pretty. But Chavez's activities were ultimately successful, and the farmworkers today have benefitted greatly.
Nobody is perfect - not Washington, Lincoln, Gandhi or King. Everyone has flaws. When we judge someone "good", it must be with this understanding.