The rail system has been stagnated in the last 40-50 years and maintenance neglected. The rail is not straight and gives a lot of lateral movement, vibration and noise. The train departments are not sound isolated or have air condition. The speed limit (as a result of the neglect) makes the trip slower than on the highway, with around 30m in favor of the car on a 3hr trip. Motion sickness is an obvious problem. Because of the lack of maintenance, there is almost never alternative rail tracks which mean that when they do maintenance they have to shutdown the whole track and switch to bus, often doubling or tripling the total traveling time. Train tickets is also about 100-200% the cost for distances above 3hrs.
I don't know if the French rail is better, through I do know the German one is. Getting into a train department with sound isolation, air condition, plenty of space and almost zero lateral movement was amazing (sample size of one trip I made between two large city centers), and if that was the option I would pick that ever time even if the train was slower or costed more. It is the 50 years old black tin can of motion sickness that makes me pick a plane over a train.