I’m experienced with rails and I rarely use scaffolding simply because my end goal is far from simple CRUD actions. However, there’s no strict rule to not use scaffolding. Some coders frown on it because it is actually scaffolding, literally. It’s not a finished product. Scaffolding supports you as you build the actual product. Search google images for “scaffolding” and you should get the idea.
Keep in mind scaffold is just one of many built-in generators in Rails. A Rails generator is simply a script that outputs some generic code. Generators are very useful time-savers, and you’ll quickly find yourself writing generators for your own custom needs.