I think it's unstable. I had to disable it because it was causing frequent reboots that wouldn't stop until the computer was powered off long enough that the memory cleared out. Even if I booted into Windows after the forced reboot, it would still continually reboot. I don't know what the issue was, but it stopped after disabling hardware acceleration.
Most likely patents. Firefox only has video acceleration on Windows if you have codecs installed already (because the codec people paid the patent license fees) and the acceleration relies entirely on the codec+driver combo.
Chrome ships libavcodec/ffmpeg bits because they've paid the license fees.