The only mechanical unit in the ADVENT Family, they are robotic not organic. Meaning, you cannot Mindcontrol them but can easily hack them. The Mecs assume the roles of the troopers late in the game, and they are essentially troopers only much stronger and does not throw grenades anymore but rather shoot micro-missiles. They usually shoot micro-missiles when your squad is lumped together, just like the Troopers, or any enemy unit for that matter.
ADVENT MECs favor going into Overwatch when revealed making them vulnerable for the initial run. They can't find cover too. They may be big, but they are so easy to hit and control.