it makes me wonder about what happens if you have to shut down an engine in flight, since it will be windmilling at a good speed for quite a while.
True, they don't like it for any extended period. We once had to send a jet back from Ascenscion Island on only 3 engines, and to stop the 4th windmilling all the way home, a big wooden pole was rammed down the engine and bolted into place.