Why is Shedinja doll "underwhelming"?

As the title says, I'm asking why people don't use this more often. It guarantees seconds worth of both frames and cc immunity, allowing it to easily negate a unite move. But I feel like the community hates it. Is there a reason why? Should it get a buff, nerf or get reworked entirely? Personally, I use Slow Smoke, but Shedninja Doll is a close second