Some devices will support it only "in software"; hitting the CPU on a busy network devices is basically "game over". Other devices support IPv6 only in a limited way, e.g. no VRF support for IPv6. Of course for those type of problems the network vendors are more than happy to sell you new hardware for $$$ with a questionable ROI. But for some features your network might rely upon all you will get back is a "IPv6 support will be added next year"...