You go from being undocumented and under the radar to being given an A number and a file with your name, address, identifying information, fingerprints, etc. In return you get a two year reprieve and a work permit. After that you risk the program ending and a new administration using that information to target it's next round of deportations. That is a damn big risk to take.
They are currently saying that any information provided will not be forwarded to ICE for immigration enforcement. However, there is the caveat that the policy may be rescinded or modified at any time.