the engine is still being spun by the momentum of the car, so your hydraulic assist on steering AND brakes will work as expected.
This is false, at least on my old '84 Plymouth Voyager back in the day. It had a loose wire in the distributor cap position sensor which caused the car to stall semi-randomly. ($20 part, labor to find it: $priceless!...)
As soon as the engine stalled, even coasting along, you lost 100% of power assist on the steering. Power assist on the brakes lived a little bit longer, but it faded with each press of the brake pedal, and you'd run out long before you were able to stop the car at even 40MPH. Had to practically stand on the brake pedal (use the seat as leverage to put most of my weight into it) to get the car to stop at that point. That and put my shoulders into the steering to try & get off the road. The problem only happened at low speed for whatever reason, so I never had to attempt stopping from highway speeds, but I can't imagine it would have been fun.
As far as engine momentum, on an automatic transmission that's almost nil. The car itself is moving forward yes, but the torque converter won't push that momentum back into the engine to keep it spinning when the ignition cuts out. Think about trying to push-start an automatic transmission (you can't). The car's momentum will keep the car itself rolling, but anything else that's driven off the engine (various vacuum and hydraulic pumps for power assist brakes, steering, suspension, etc.) is gone within a second or two. Long before you're safely stopped at any rate. I'm not as up on torque converter operation as I might be (software geek), but I think as soon as the engine-side isn't pushing on it, you essentially drop back to neutral. There's no way for it to run "backwards" with torque pushed from the car's wheels.
I'm going to go out on a limb and guess the "hardened criminal" type they're undoubtedly going to reassure us all this would only ever be used against probably has enough experience driving to safely control a car in this state. A younger or less experienced driver could easily panic at highway speeds, and I doubt the result would be all that Safe of a Stop.
Given the progression of how Tasers have been used, it seems inevitable this will be used against an increasing number of drivers. These days you look at a cop wrong, and he's unsnapping his Taser from his belt. I doubt it will be terribly long before this technology is used against an increasing number of drivers.
The result will likely be accidents or deaths, quite probably of innocent bystanders in the path of uncontrolled vehicles. All of which will be blamed on the driver, even though it's probable few of those deaths would have occurred had police not intervened. Not unlike how so many cases of Taser deaths are swept under the rug with the assurance that it's "less lethal" and therefore reasonable to use it against a wide range of innocent-until-proven-guilty people even where deadly force per-se would have been considered unacceptable.
Personally, unless a shoot-to-kill is merited, I don't see how it's acceptable to employ techniques that can so easily end in death. It'd be nice if law enforcement had to take the Hippocratic Oath.