I have found two challenges of creating a pinhole camera with short focal length (by focal length, I mean the distance from the pinhole to the film):
Pinhole size: Short focal length means we need a small pinhole, which is hard to make.
Vignetting: The light has to travel further to the edges of the film (compared to the center) which means there will be vignetting.
Are there any more effects I haven't understood? Does the fact that the light has to travel further to the edges of the film (compared to the center) cause any aberrations etc?