BTRFS is so mature already, I never lost my data with it
Dude, nobody said BTRFS is mature. Did you read the part where I've had to manually rebalance several volumes on multiple occasions? I'm sorry that you interpreted this statement as a ringing endorsement of a mature filesystem - but it's not the case that users should have to do this kind of babysitting in a mature technology.
I *have* had BTRFS fill my logs with checksum failures on a couple of dying disks, and I was able to recover everything intact (the bulk of this data had shasums thanks to some deduplication I had been doing months earlier).ext4 on the other hand (by its very design, unless you count recent kernels where metadata may be checksummed) happily allows the disk (or whatever) to take a shit all over your data without so much as the slightest hint that something might be wrong until you go to open a file years later and discover it's zero bytes long, truncated, or full of garbage.
The data integrity features of the new file systems are nice only if you can assume them to be bug free.
No shit. But if your idea of data integrity is to start with something that doesn't even try, there just isn't any hope of that is there?