The problem I have with a one piece flat headstock (especially on ERGs) is the right downforce at the nut. The sharp break angle always seems to cause the tuning to be "grabby" for lack of a better word.
With an angled headstock the string has one friction point. The nut.
With a flat headstock the string has 2 friction points (the nut and the retainer) and a very sharp angle . On the really large strings the large winds like to get stuck as they pass through the angles...
There are work arounds, rollers, making sure everything is rounded and smooth. The problem with the retainers is that you need to have a large enough radius on the friction point so that the string always has more than 2 winds touching for smooth tuning.