The recline-blocking device has made the headlines recently.
One thing that many didn't consider is the risk the device poses in the case of an incident.
It would all come down to whether or not the measurement for passing Head Injury Criteria is violated with the installation of the lock-out device. If it is—and this would have to be demonstrated on every seat that is currently in service, and will be in the future certified to be HIC compliant—then the current position taken by the FAA [may constitute] allowing HIC requirements to be violated, when it is their requirement to begin with.
The device has clearly not be tested—seats are thoroughly tested to assess if they pose any risk of injury to the head. Not only could your head hit the device in case of impact, but the seat itself might lose some of its needed flexibility.
Skift reports that the FAA, the UKCAA and EASA seems not to really care at this point, leaving the decision to airlines.
With the potential massive liability costs incurred after an accident though, I don't believe many airlines will continue to allow the device for long.