Meliodas's Full Counter is a demon ability that reflects magical attacks, but it has clear limits people often overlook. It only works on direct, energy-based magic thrown at him, so physical strikes, curses, or traps go right past it. It also requires him to react and swing the attack back, so if someone hits him too fast or too many times at once, he can get overwhelmed.
In the anime, there's a whole episode where Estarossa basically shows how it fails—he just punched Meliodas, no magic involved, and it did nothing. So for pure mages, it's terrifying, but against any hybrid warrior or someone smart enough to switch tactics, it's far from an 'I win' button. Honestly, I think the series nerfed it later on purpose, 'cause otherwise every fight would be boring.