How much safer your data is when using BTRFS over Ext4?

BTRFS does provide file and metadata checksumming which is not available in EXT4. That alone should make your data safer. BTRFS also enables functions such as snapshots. That will protect your data from a major cause of data loss - human error. BTRFS and ZFS are the only two major file systems that can do this. NETGEAR works very closely with both file system communities and shares our improvements back to the community.

I don't have any numbers on how often EXT4 would write the wrong data if you are looking for statistics.

Here is a published article on the state of file systems. You'll definitely see more BTRFS enhancements over time.
http://arstechnica.com/information-tech ... lesystems/