I suspect the slowness and the suddenness of a "you owe us X" is pretty stressful for many people. But if you read the letters it is mostly "hey send us some stuff so we can look at it" and while drawn out.... my experiences were fair and completed normally (granted that is just one experience).

At one point I printed the wrong thing entirely... they just sent a letter telling me I sent them the wrong thing and the clock kept restarting.