BTRFS does have stable, usable multi-disk support. The RAID 0, 1, and 10 modes are fine. I've been using BTRFS RAID1 for over a decade and across numerous disk failures. It's by far the best solution for building a durable array on my home server stuffed full of a random assortment of disks—ZFS will never have the flexibility to be useful with mismatched capacities like this. It's only the parity RAID modes that BTRFS lacks, and that's a real disadvantage but is hardly the whole story.