The disadvantages of using authentic texts in the language learning classroom

It's probably idiosyncratic
My theory for why using authentic texts with language levels of all learners has been such a selling point over the years is simply that the words that are used to describe what are commonly taken to be the two options leaves one option in an unarguably strong position – the two words being “authentic” and its indefensible opposite “inauthentic”. In fact, though, the two good options a teacher has are usually to choose an “authentic” text or a more “representative” text.
We try to choose between the hundreds of possible language points we could cover in order to tackle the most important and manageable first. In the same way, a graded text is rewritten not just to be simpler but also so that the language is the kind of generally used thing that students need in order to be able to communicate in the greatest number of typical situations, i.e. to make the language “representative” of the English language as it is generally used. This is not the case in most authentic texts, where the skill of a writer is often to make their use of language personal and therefore unrepresentative of how other people use English. Even when the individual writer hasn’t stamped their mark on the text too much, you might also have problems dealing with the idiosyncrasies of particular genres or ways that particular nationalities of native speaker write.
These points can be great to look at with very advanced learners and can be exactly what they need in order to show them that there is still a lot to learn in English. Needless to say, the last thing that will motivate an Intermediate student is to be told how much there still is to learn! These idiosyncrasies are often taken out of graded texts (which is the main thing that makes them so dull for native speakers, more so than the simplification of language) and it is possible to partly do the same with authentic texts. One hint is to avoid famous writers and just go for almost miscellaneous stuff like shorter newspaper articles.
The grading of the various parts of the text might be different
This can particularly be a problem with novels and poetically written magazine articles, where the descriptive introduction is often several levels higher than the story will be once the plot and/ or dialogue starts. The same is true of punning newspaper headlines. Approaches include giving the difficult parts in summary form and just using an extract from the original text, or doing activities just with the easy bits like the captions or dialogue.
The information can quickly become out of date
This can be a problem both for student, for whom the language might fly out of their heads at the same time as the information gets replaced with something more important. It can also be an issue for the teacher, who might have spent lots of time preparing the pre-teach and comprehension questions only to have to throw the text away after a couple of days. This is mainly a problem for newspaper news stories, so there is no reason why you shouldn’t use more long-lasting formats like magazine articles, newspaper articles with more analysis, fiction or biography instead.
The difficulty can put people off reading
For some people the challenge and achievement of reaching the end of an authentic text for the first time is just the boost to their motivation that they need, even if they then don’t touch another authentic text until they have managed to reach a more advanced level. For other people, however, the struggle of dealing with authentic texts can just convince them that reading in English will never be worth the effort. The most common response to this from teachers and teacher’s books is to give students simple general comprehension and skimming and scanning tasks, and to skip the detailed comprehension tasks. This can work and give students a sense of achievement, but some students can feel it is just a con job to make them think they have understood when they haven’t really, especially if you try this trick a few times. You could try your best to choose the easiest authentic text you can find, but with a student or class that doesn’t like a challenge it is probably best just to stick to graded texts.
The vocabulary is not graded
However easy an authentic text you have managed to find, it is unlikely that every word in it is one of those “most used words in English” that are marked in learner’s dictionaries. This is particularly the case with children’s books, which can be easy and fun for adults to read but often have a vocabulary that is more suitable for the under 10s, and in which the most useless words are often those which are repeated the most often. This can be yet another good opportunity for students to test their guessing vocabulary from context skills. Alternatively, you can provide a glossary to the words you are not expecting them to know at that level but are vital for understanding that particular text, something that is sometimes given in graded readers and even test readings. Another technique is to underline the words that are probably new to them that you actually think are useful, so that when they get busy with their dictionaries in class or at home you know they will be somewhat guided in what they learn.
The grammar is not graded
The grading of grammar in a text is usually more difficult to spot and easier to forget about than the grading of vocabulary, but in a graded reader the writers are even more careful about the grammar than the vocabulary. This does not necessarily mean that all the grammar has to be exactly the same as they have already covered in their books, as grammar is easier to understand than produce and seeing it in context for some time before they tackle it in class will make it easier for them to pick up. A good rule of thumb is that most of the grammar in the text should be what they have already studied, and most of the more difficult grammar should be within one level (e.g. halfway through the Intermediate level textbook if they are halfway through the Pre-Intermediate level) and guessable from context. If there is any grammar that is even higher level, you can try and get the students to ignore it by having the comprehension tasks only for the information elsewhere in the text, or providing a “grammar glossary” similar to a vocab glossary.
The two surest ways of checking that most of the grammar is of the right level are using graded texts and rewriting authentic texts. Another possibility is just to use a short passage from an authentic text that only has the right kinds of grammar in it. If you do want to search for an authentic text that has the right kind of grammar, one way of searching is by genre. For example, stories usually have Past Perfect, Past Continuous and Past Simple, but jokes and anecdotes might use present tenses instead.