FEAR stands for Focus Sash Endeavor Rattata, which is quite a good strategy, especially if the format has disallowed dynamaxing. But this strategy has it's own weaknesses, so how viable is it?
FEAR is pretty unviable.
Any hazards, switching, or ghost types can counter it, as well as multi hit moves and weather. There are so many things that can go wrong to the point where it is unviable.