There's absolutely nothing wrong with btrfs and I am actually sad Jolla decided to drop it.
In reality btrfs is fast, scalable, effcient and reliable, providing a good set of modern well-thought-of features.

Any and all FS'es run into trouble when users pump their partitions too full, there just needs to be enough leeway in the system.
My n9 had 64GB of storage which was enough at that time, what, almost 5 years ago. All the btrfs problems on Jolla stem from the laughable/cryable 16GB of storage....
In 5 years device minimum allowed storage size should have grown like Moore's law, making something like 256GB the norm now...