The MG Team are the ranged unit of the Postmodern Era. In contrast to progressive and modern era ranged units, MG teams are resistant to fast units and weak to artillery units. They have the special ability Contact! which allows them to retaliate against any unit that attacks it from within its range (7 hexes on the postmodern map), including those that are hidden.
Though its description says always retaliates against attacks, this refers to attacks that are in range including attacking stealth units that are hidden in terrain. There is no retaliation if the attacking unit is outside the range of the MG Team unit.
The currently available units that can attack a MG team without retaliation are artillery units, Snipers, ranged units from Tomorrow Era upward (except Surrogate Soldier and Ninja), Combat Drones, light units with Dragon Breath ability, Hover Tanks, Battle Fortresses, Behemoths and Ronin Bots.