Interesting point about the mass being in front of the steering axis. I wonder how this changes as you move to a shorter stem. Presumably it makes the bike less stable.
Spot on. Bikes with shorter stems will have more direct steering, ie twitchier than those with a long stem. Similarly, bikes with a steep head angle are also quicker steering. Combine the two and you have something that will fly around multiple tight corners but will but the fear of god into you on long, fast descents.
The trend to short stems is off the back of the latest “Enduro” bikes with slacker geometry and longer tts. The only way to stop them steering like barges is to put on a very short stem. FWIW I like these bikes, but my old Turner Flux was the fastest bike I’ve ridden on tight, switchbacky singletrack and it got seriously twitchy once you went below a 90mm stem