The Merc has plenty of survivability via mobility(Hydraulic Overrides), heavy armor, shield, electronet, hard stun, kolto overload etc. to avoid being focused. Sure in yolo you'll get the tunnelers, but they usually lose. Once the Jugg jumps in, hes got one other out being guardian leap. Hes most likely going to be slowed and/or stunned after unstoppable wears off. Hes got some tools to help him survive, but ED was definitely needed to help him survive the hard stun + burst.