Care to substantiate that statement? It seems rather arbitrary to just say that it's garbage when it is running and has been running successfully for the vast majority of its users. It also offers two features that ZFS does not: the ability to grow a pool, and offline duplication.
Why should it matter? It's an extremely niche technology that's only interesting to some home users. I see no reasons why other users should care about a RAID level they're not interested in.
(I don't use btrfs or any other COW filesystem because of significantly worse performance with some kinds of workloads, but it has nothing to do with maturity of any of them.)
Yes but you will lose data if you are writing to your array when the power goes out. RAIDZ (ZFS) does not have this problem. See BTRFS RAID5 write hole.