I've been learning Italian for some years (at present my level is C1): during that time I've been essentially a self-learner, with the exception of a few months, and I've been a quite active user of Italian.SE.
I've always tried to benefit from extensive reading, so I've post many questions based on the books I've read in the above mentioned SE site.
The fact is that, from time to time, I've received comments to such questions that advice me to avoid certain authors, claiming that I may acquire bad habits as a consequence of such reading or that they can be harmful to my vocabulary and grammar acquisition. I've also seen this kind of advice when reading the opinions about some novels written by some (supposedly) language learners. Basically authors that use regional vocabulary and structures in their works, that tend to invent some words or that make some characters to speak with a mixture of Italian and another language (usually called "dialect") from Italy. To give an example of a couple of well known authors, I can mention Dario Fo and Andrea Camilleri, but there are many others.
I never took such comments seriously and I've never had the impression of acquiring bad habits because of such reading (well, it could be that I've sometimes make the mistake of using a regional Italian word when I was not supposed to do so). In addition, I've tried to argue that C1-level exam requires you to show some understanding of real regional colloquial Italian (which is frequently the mixture of Italian and "dialect" I mentioned before), at least it was that way for the exam I took. But it seems that some people don't believe me and, even now, insist on giving me this kind of advice.
Anyway, I wonder if there is any evidence based on research that reading certain types of texts can be harmful to foreign language acquisition (maybe at lower levels).