.... What is your goal? A power supply for your projects that will be reliable and accurate, and safe; or is it to learn how power supplies work and how to build them?
You made the point: For sure it's more expensive (mainly if you calculate the time) to build it yourself (but
don't tell it my wife, please
), and you end up with something not so performant than the same thing out of the box, but the reason why I'm playing with the Arduino is that I love the learning effect, and I have to admit that most of my projects becomes boring, just when they are finished. The journey is the reward.