Le Passage du Gois, France
This is one unique road since it is, quite literally, in the ocean. There are even a couple of hours in a day when you cannot go through the road. When it is high tide, you should look for another route.
Patiopoulo-Perdikaki Road
It is difficult to maintain the Patiopoulo-Perdikaki Road because it is so long and out of the way. You will see a lot of potholes here. Parts of the road have even fallen off. It is a scenic drive but also very stressful.