Dive brief: 

• At an Advertising Week panel Wednesday, CEOs from Publicis Omnicom competitors questioned the future of the largest merger in agency history. 
• CEOs from Havas and JWT were skeptical the merger would be successful, citing its massive size and two CEOs with conflicting personalities as major concerns. 
• Ross Levinsohn, CEO of Guggenheim Digital Media, didn't predict failure, but rather said the new mega-agency will survive only if it has "ruthless execution." 

Dive insight: 

The executives have reason to be wary of a merger of this unprecedented size. Their negative comments could come out of fear because they are rivals or could be a genuine concern for the future of the agency. Like Levinsohn pointed out, huge size isn't necessarily a recipe for disaster—just look at Google.