Don't you have codes and license requirements for electricians and roofers? In my country, most of what makes electricianing harder than electronicsing is the rules. There are rules about how close you can put mains and low voltage wires, which your guy should have followed even if he didn't know why, rules about what parts of the house you can fix them to, or if they can be laid loosely or must be stapled to the framing, the spacing between the staples, how to pull a wire through a hole, where you can drill holes for wires, using a wrong colored wire with a sleeve on the end, etc. All the understanding of electrical engineering in the world won't have you knowing those rules.
The small subset of codes in my area that I personally looked at were significantly out of date and it was unlikely that they would be updated anytime soon. There are state codes which were out of date for a long time but they just recently updated. However there are also local codes which are not up to date. When they're not in direct conflict you need to follow both, if they are then technically the state should take precedence, but if you look into it then you'll find that the locality doesn't care and will fine you and it's up to you to defend it which will be expensive and you could still lose. The codes are only as good as the people maintaining them and it seems that the maintenance is not a priority. This was not for electrical - perhaps that one is given more attention.
And then you have old houses with all sorts of weird wiring and constraints and the contractor/electrician works around those and does things mostly to current code the best they can.
