My KindShock adjustable seatpost has the keyways to keep its side to side play at a miniumum. Seems to work pretty well, but a fork and a seat post dont see the same kind of forces.
It will be interesting to see how the system holds up long term. X fusion seems pretty confident in it, But that doesnt mean much. Im sure every product that has had problems was released with a lot of confidence. I personally hope that it works great, and ends up being a game changer for inverted mountain bike forks. Only time will tell.
Keep in mind that motocross upside down forks have double crowns. Its not like they are giving up a crown when they go upside down.
Also pretty bomber axles, my Honda with USD fork had a beefy steel axle, locked with nuts. Maybe the 20mm axle diameter, and pretty much standard quick releases are the problem for seeing those forks on bikes.