I've been using bcache (SSD) with btrfs (4TB hard drive) for while, and am in the process of ripping it out. What I found is the filesystem gets wedged if there is a heavy write load (lots of files) such as running debootstrap. This happened with both write-through and write-back. The symptoms are all requests stall and a kernel log message about hung btrfs tasks. Only a reboot (typically needing sysrq keys to complete) works.

The problem did not occur previously when using btrfs without bcache and the same workload. It also doesn't occur during "lighter" workloads.