I'd say scanline rendering is "more of a hack" than ray tracing, but both can produce useful looking results, of course. They are both crude approximations of reality, but ray tracing is a closer model and it makes non-local phenomena easier to model (especially if you do things in a physically-correct way, respecting the law of conservation of energy etc.) Arguably, you would end up writing a "Universe simulator" if you wanted to accurately model absolutely everything that's happening in the real world :)

By the way, I've written about this exact same topic in my initial ray tracing post:

http://blog.johnnovak.net/2016/04/28/the-nim-raytracer-proje...