If you don't have an engineers certificate for every modification, you won't pass for registration. No registration on a vehicle means no use on public roads. The police now have number plate scanning devices on their vehicles too that scan while they are driving and alert them to registration status on passing vehicles. The fines and penalties for using an unregistered vehicle on public roads are more than the registration costs. So not worth it.
As I mentioned, each year our vehicles in NSW need to pass an inspection to be cleared for the following years registration. If the inspector, most mechanical shops, is inspecting a non-original car and questions the modifications, you need to provide the engineers certificate to prove those modifications are certified and legal. The same if you were pulled over by the police and they questioned you over modifications, you need to carry the engineers certificate to prove it. This is where the police persons knowledge and attitude can become a huge grey area. They have the right to fine or take the vehicle off the road but there are so many stories of misinformed police officers not knowing the rules but using their status to do so. I've been on the receiving end of both positive and negative too. Nothing against the police, just some people in general are a-holes.
Other states are different. Some are like us that require yearly inspections, some only require them when a vehicle changes ownership. But in those states, the modification rules still apply. If you were pulled over and the police detect uncertified modifications, they take the vehicle off the road until it is rectified. Which either means change the modification what ever it may be back to factory standard and have the vehicle inspected and cleared. Or have the modification engineer certificated.
Now, more grey areas. Each engineer and registration inspector also has their own interpretation of the rules. It's a matter of finding and using one that you get along with or are both on the same page, that makes it much easier.