I can balance my boat well enough that it will sail for a few hours at a times without tweaking in the correct conditions. Gusty conditions or very large waves can make it impossible. Another thing that plays into it is point of sail, trying to be close hauled or on a dead run is really hard for me to balance the boat at.
The key is to make a stable system so that whenever the wind
angle deviates, the forces want to turn the boat back towards where you want it to be. This is relatively easy on a lot of ketches since you can sheet the mizzen and jib
so that if the boat is falling off, the mizzen overpowers the jib
and pushes the bow back into the wind but if the boat is heading up, the mizzen depowers (often through a little luffing) and the boat falls off again. There is more to it than just this (you can use rudder angle similar to heaving to and a few other tricks) but that is the basic idea of how to create a stable system.
Anyone who claims that the boat will steer within 5 degrees for hours on end is a liar but you can get the boat to average out to a very desirable course and averaging out to within 5 degrees is totally reasonable in good conditions. There are many examples of people going around the world like this. I believe that Robin Knox Johnston sailed a significant part of the Golden Globe without a working windvane
It works well enough for me that I haven't bought an autopilot or windvane yet. This guy isn't totally off his rocker.