I only skimmed this but if I understand correctly the CPU needs to be in debug mode to access these instructions right? If so then it's hard to see why the authors present these instructions as dangerous - I imagine once the CPU is in debug mode you would have unfettered access to the system even without these instructions.
(not that it isn't still interesting!)
The undocumented instructions pdf paradox
Not sure if it's the same researcher, but I remember some talk about using fuzzing to find undocumented instructions on x86.