I lost enough Btrfs m=d=s=RAID5 filesystems in past experiments (I
didn't try using RAID5 for metadata and system chunks in the last few
years) to faulty SATA cables + hotplug enabled SATA controllers (where
a disk could disappear and reappear "as the wind blew"). Since then, I
made a habit of always disabling hotplug for all SATA disks involved
with Btrfs, even those with m=d=s=single profile (and I never desired
to built multi-devices filesystems from USB attached disks anyway but
this is good reason for me to explicitly avoid that).

I am not sure if other RAID profiles are affected in a similar way or
it's just RAID56. (Well, I mean RAID0 is obviously toast and RAID1/10
will obviously get degraded but I am not sure if it's possible to
re-sync RAID1/10 with a simple balance [possibly even without
remounting and doing manual device delete/add?] or the filesystem has
to be recreated from scratch [like RAID5].)

I think this hotplug problem is an entirely different issue from the
RAID56-scrub race-conditions (which are now considered fixed in linux
4.12) and nobody is currently working on this (if it's RAID56-only
then I don't expect it anytime soon [think years]).