This post makes no sense.

Also, what do you think happens when a cog unthreads? It just bounces around between the hub and the chainstay while you slow down with your brakes. How is this dangerous?

It's much more dangerous when the cog cannot unthread and the back wheel locks up after sucking the chain into the drive chain.

The only problem with freewheel hubs and fixed cogs is that you loose a reliable braking mechanism. Back-pressure is fine but full-skidding is unreliable.

Other than that, it's the same.