I don't know about GA aeroplanes these days, but I know of one airliner where the systems are so integrated that the inter relationship between systems that at first (and second, third and fourth) glance appear totally unrelated are in fact intrinsically entwined to make simple mods almost impossible.
Clock signals control which computer is primary on a given date when the aeroplane wakes up in the morning, weight on wheels messages on the data bus drive changes to pressurisation controllers, resetting a clock can make the flaps fail....
A quick and dirty assessment may be a big mistake, particularly if someone else installing an earlier mod has taken a required input from somewhere unexpected because it was convenient.
Probably less of an issue in a "classic" aeroplane like an old Cessna or piper, but newer machines with glass instruments or ones with post delivery installations of gps etc might fall into the danger zone....
I am not an engineer but I would think that any sort of installation/approval would require a detailed look at the history of the particular airframe to make sure that they aren't opening Pandora's box