a one-trick pony
A person, group, or thing that is known for or limited to only one unique or noteworthy skill, talent, ability, quality, area of success, etc. The app developers took the world by storm with an incredibly addictive game for smartphones, but they've really been seen as just a one-trick pony ever since that breakthrough success. As most readers suspected, the famed fantasy writer turned out to be a bit of a one-trick pony, genre-wise, with his debut novel in literary fiction being universally panned by critics.