Growth lessons from agency mergers and acquisitions

Given that agency mergers and acquisitions (M&As) often fail to deliver, would remembering some basic principles make success more likely? 
Disciplines are certainly blurring, most obviously in the bunfight-cum-love-in between consultants, service designers and ad agencies. So it’s hardly surprising mergers and acquisitions are all the rage. 
Most of it pivots on Accenture’s hoovering spree - from their most recent purchase, Aussie hotshop The Monkeys, to spec