An underlying problem with any post manufacturing mod is hull applicability.
The STC OEM, regardless of whether that might be the aircraft OEM or, more usually, some other enterprise generally has NO idea what other mods may have been performed on the hull.
As a strict consequence, the STC can ONLY apply, at the design and certification stage, to the aircraft OEM standard aircraft serial range for which the STC OEM investigated applicability.
The result is that the installer (preferably with some formal design input/assessment) has to check and be satisfied that there are no other mods on the hull which might cause a problem with the STC.
This is why, in States which so permit, it is a lot easier to run up a mod design for only a few aircraft which the designer is able to check personally for such incompatibility.
Overall, the STC attests to the suitability of a mod for a given range of standard aircraft while the installation certification documentation (which varies from State to State) covers the specific hull considerations.