Maybe not on 1 billion devices, but ZFS was running on mission-critical Sun/Oracle hardware with expensive support contracts for many years. ZFS is currently probably the most reliable modern filesystem.

Still, rolling out APFS on such a large number of iDevices/Macs is a very impressive feat (although APFS is, of course, a simpler filesystem). The problems are with many of the 'edge cases' (RAID, etc.). This is not really surprising, btrfs has been in development since 2007 and still has problems with more complex setups.