I have seen people using the expression "hard to read at spots" for stating that some parts of a text are unclear, or that some reading conditions are negatively affecting the understanding of the text. However, I couldn't find more information about it on my search for the expression, excepting that "at spots" may also be used alone to represent "in some places", which suggests "at spots" is the real expression that I'm questioning here.
Regardless, I am only concerned about the formality of stating that "____ is hard to read at spots". Could it be used in formal writing, such as in academia? Or should I say "in spots" or "in places" instead?