My problem with BTRFS and ZFS, and I admit I may be in the minority, is the handling of RAID. Creating a raid setup is fantasically easy in ZFS and BTRFS and is miles ahead of mdadm. However, the problem comes when you want to expand your raid. If you want to increase your capacity, you have to create a whole new raid the same size as your old raid.
I'm sorry, but I really don't want to put together another 16 TB of disks and add another 16TB to my raid. I just want to add another 3 or 6 TB hard drive and expand it that much. I don't consume TB of data in the span of a few weeks or month. Adding an additional 3 TB to my RAID will last me for another year or so. It would be pointless to add another 16 TB and it would waste 2 additional disks for no reason.
If I add another 3 TB disk to my RAID6 under mdadm, I get another ~3 TB. If I add another 8 3 TB disks to my current raid, I get another 24 TB. If I add anothe 8 disks to a ZFS or BTRFS raid setup, I get another... 16TB. Fuck that.
Other than that, I haven't found anything that I dislike about ZFS or BTRFS... but the RAID situation is a real deal killer.