When one learns how to draw, what they are really learning is the ability to see more clearly and communicate what they see or can imagine. As such, drawing is a tool for study, and communication even before it is a tool for making art. I just can't seem to get my characters anatomically right. I saw your booklist on your site. Also, draw from live nude models as much as possible, that's the best method to learn it.
Nothing is really harder or easier to draw in a mechanical sense.
If a person can draw one thing, they are perfectly capable of drawing something else with the same degree of accuracy. The reason people, and especially faces, are so difficult to draw is not because they themselves are any harder to draw than a tree. What changes is our tolerance as viewers for any degree of inaccuracy. We are so intimately familiar with faces and their associated nuances, that the slightest deviation appears as a gross misproportion. Because of this, I am constantly demanding my students draw people, and especially self-portraits, because they effectively highlight any weak skill areas that need attention.