The whole language approach to learning how to read is the opposite the phonics method that taught generations of children the basics of the English language. With phonics, children learn the sound and inflections of each letter and how to sound out words with which they may not be familiar. In the whole language approach, children have no clue what a word or letter should sound like. They are supposed to see a word and repeat it until they have it memorized and can recognize it among other words. With each word, their pool of words grows, but with this approach each word is an individual and separate item to be remembered. This is unlike the phonics approach where language is built brick-by-brick, and the child learns different sounds and letters with the ability to reconstruct these into a variety of words. Children with limited ability to memorize a sequence of words are at a disadvantage with a whole language approach.
The whole language approach assumes a child assimilates key information through the eyes. But not all children are visual learners. Asking children who are primarily auditory or kinesthetic learners to figure out words in context to other words impedes their ability to learn the language and to read. Whole language does ask children to write, assisting kinesthetic learners, but writing comes only after a basic understanding of language as other than a series of memorized words with meanings. Visual children pick up clues and may figure out words and their meanings from context. Even so, learning solely on visual clues still leaves students without a basic understanding of how to sound out and pronounce a word they don't know, not just how to figure out a meaning. Many times meaning may not be deciphered through context.
The whole language approach places a heavy burden on students with other problems such as dyslexia. These students already have problems making sense of what they're seeing on the page. Yet with the whole language approach they're asked, without a basic understanding of how words are made, to figure out words in context. For these students words aren't in the correct order, and they jump around on the page. This makes it difficult for them to figure out meaning from context.
The whole language approach used alone doesn't provide the basic tools students need to obtain higher levels of comprehension. They learn how to decipher meaning from context, but that context is contemporary American English. When faced with writings from Thomas Aquinas, Aristotle or the King James Bible, students have no basis on which to understand words that are archaic and used in centuries-old sentence structures.