One major disadvantage for this method is that is works on the assumption that a second language is learnt exactly the same way as the first. Second language acquisition varies considerably from first language acquisition.
Another criticism of the direct method is that it was hard for public schools to integrate it. In his book, R. Brown (1994:56) explains that the direct method wasn’t successful in public schools because of “constraints of budget, classroom size, time, and teacher background (native speakers or native like fluency) made such a method difficult to use.”