Then you have the Pyrenees between the Iberian peninsula and France. Aside from being mountainous, also fairly low population density, and rather poorer regions to the point that you'll find abandoned villages.
So, the only sensible connections are along the Mediterranean (Catalonia) or the Atlantic coast (Basque Country).
Another reason is that Spain has high-speed rails only since the nineties.
IRC, just in the recent years, there was a high-speed connection built to the Basque Country (from Madrid). From the Basque Country you can go to France with a light rail.
So, the only high-speed connection is along the Mediterranean.
In a way, it's a center/periphery problem.
France is prioritising connections from/to Paris, Spain from/to Madrid.
Unless either side improves the network at their side of the border, it also makes limited sense to improve yours.
Subvolumes: "Lightweight partitions" allow you more easily to define the scope of a snapshot. With ext4+lvm you have to create a new partition and logical volume.
I don't think, it is a big game-changer, but then I don't think, the other things are such dealbreakers as many people think. I consider "is more prone to data corruption" as largely historical/anecdotal.
Btrfs send sounds appealing, but personally, I prefer my backup solution to be independent of the filesystem.