The wing doesnt seal-up the holes that you drill into the trunk and it leaks, it is junk
Instead of putting a nut on the end of the stud thats casted into the bottom of the OEM STi wing(smart), the replica wing you screw the bolts into the bottom of the wing.
Theoretically its shouldnt matter but theres a reason why subaru chose to do things they way they did, because they engineered it and it works.
It really doesn matter who installs it, the manufacture of the replica overlooked this very important detail for whatever reason (usually cost, but could be lack of experience, or many other things).
That being said you can buy the wing and 'make it work' but it might take some more time and a couple tubes of silicone or whatever ghetto solution you can think of to seal the holes.
Goodluck!