This is true,
but i did this already 4 times 
and succeeded every time
It is possible to convert from ext4 and from reiserfs. And it is possible to get a clean state afterwards (This is described in detail in the btrfs-Wiki at kernel.org)
The disk should not be to full if you want to succeed, because btrfs needs some space to breath. Optimal is less than 50% full. Maximum may be 75%. In this process i would suggest to use a second drive(!), and switch to RAID1 with btrfs directly after converting.
You can reach a clean state after
- deleting the ext4_saved snapshot and then
- after a balance
(search for RAID1)
But i would also suggest to have a backup from your valuable data beforehand (better safe than sorry)
This is written in german, but i may write the same in english soon
And later you can make good backups with btrfs send/receive
