That's exactly why I say that different viewpoints give different concept of easiness. To me there are a lot of way to find minima (or, at least, critical point

That's exactly why I say that different viewpoints give different concept of easiness. To me there are a lot of way to find minima (or, at least, critical points) of a functional: the direct method in CV, mountain pass theorems, etc. Most of them just need to know very general information about your space and functional and are actually even simple to visualize.