If not, does it mean that Btrfs in RAID mode is fully resistant to power failure?
Yes, unfortunately btrfs RAID5/6 still suffers from the write hole (10/2015).
The one missing piece, from a reliability point of view, is that it is still vulnerable to the parity RAID "write hole", where a partial write as a result of a power failure may result in inconsistent parity data.
https://btrfs.wiki.kernel.org/index.php/RAID56
However, if you're reading this in the future, this may already be fixed. (Btw. ZFS does not suffer from this issue.)
From what I've read on Btrfs, the nature of the filesystem itself makes it resistant against power failures in most situations, especially in a RAID configuration. I say most because there's always a situation that could cause data loss (say, power outage results in a spike when power comes back online, toasting your disk(s).)
