Generally speaking, people are the most difficult to draw for the following reasons:
There are hundreds of muscles and bones in the human body. The understanding and description of muscles and bones is the most complicated, and the requirements for modeling, black, white and gray, and color expression are extremely high. Of course, it doesn't mean that other things are simple, but relatively speaking, at least modeling will not be as complicated as characters, and turning over will not be as much as human muscles. Accordingly, color expression will be easier.
For example, apples are spheres, but people's faces are not as simple as spheres. Zygomatic muscle, orbicularis oris muscle, orbicularis oculi muscle, etc. And many details such as nose and lip angle need to be explained clearly, which requires great skill. People who draw still life and flowers and birds may not be able to draw good people!