United Labor Woes Damp Promise of $1.2 Billion Merger Gain

United Continental Holdings Inc. is struggling to unify two union workforces more than a year after the $3.47 billion all-stock combination that created the world’s largest airline.

Chief Executive Officer Jeff Smisek accused some pilot union leaders yesterday of “shameful” complaints about flight safety, two days after the group sent a 105-page letter to Congress saying that a lack of training on new procedures was “potentially dangerous.”