I found using mirrored vdevs in ZFS much easier to manage and much more stable.
That's not exactly a fair comparison. If you restricted your usage of btrfs to a similarly narrow range of features, you would probably have had a much better experience.
I can't say for sure that this never happens, but that's certainly not been the failure mode for any of the drive failures my btrfs RAID1 has experienced. I don't think I've ever needed to reboot or even remount my filesystem, just replace the failed drive (physically, then in software). But I always have more than two drives in the filesystem, so a single drive failure only puts a fraction of my data at risk, not everything.
That's not correct, you can rebuild online. The readonly mode is only relevant when you reboot during the failure and don't have the right options set on the volume.
But you totally can replace a live drive without affecting the availability.