It almost looks like filesystem development doesn't fit Linux kernel development style of iterating constantly and evolving with time. btrfs has had the same time as zfs had in-house at Oracle before it was declared publicly stable, and there are still buggy/unfinished corners. If you look at past successful Linux filesystems it's either an existing design that was generally amenable to certain extensions and has evolved in-tree or came designed and implemented from a different platform (JFS, XFS, quite a few more). EXT4 is the reliable workhorse if inodes aren't a problem and you don't mind time to allocate and upper bounds thereof. It evolved step by step from EXT2 to EXT3 to EXT4, all the while having stable core features and experimental features. btrfs is busy implementing features promised 10 years ago and there are bugs in regular use if you're not careful. Developing a filesystem is hard and there's no room for mistakes. Most productive filesystem development is happening in XFS and EXT4 teams with the former being in a nice stable maintenance mode. If you haven't tried XFS in the last 3 years, give it a test run. The old issues of being optimized for certain workloads have been fixed years ago. It's a good replacement for EXT4 if you need its features and tools.
