Medical devices I think should need a someone well versed to work on it.
With cars, the current model most states in the US have is a good middle ground. You can do whatever you want to your car, but it needs to pass a safety inspection every 2 years to drive it legally.
The inspections in my state are fairly comprehensive. Airbags, seat belts, headlight brightness, and structural stability of the frame to name a few.
It also helps the US has a strong car culture with tons of experienced DIY-ers, which I imagine helps.