They’re not practical because they haven’t had a reason to be mass produced, so they haven’t gotten cheap, or had the kind of effort put toward them to make them smaller, lighter and more efficient. That won’t happen until the discs themselves work.
Also, permanent magnets would reduce power consumption.
Yep turbines make a pretty impressive mess when they fail,maybe flywheels however important arent good for this app-anyway the ICE as we know it wouldnt have got off the ground(ever try to start a lawnmower without a blade that had a lightweight flywheel as opposed to a heavy one)?-Kevin