The hypothesis claims that early English was significantly influenced by the Brythonic language due to imperfect language acquisition and this influence is consistent with modern language contact theories. However, arguments promoting the hypothesis have often been faulty and the Brythonic and Germanic evidence only superficially analysed. Closer attention shows that, despite the claims, the Old English dual paradigm of 'b' is not a valid candidate for Brythonic influence. Furthermore, while periphrastic 'do' and progressive 'be -ing' may have been partly influenced by contact with Brythonic, the parallels in Middle Welsh have been oversimplified and those in other Germanic languages have been undervalued. Arguments for the loss of the external possessor and the merging of the emphatic and reflexive pronouns in '-self' are likewise problematic.

The Celtic Hypothesis provides important explanations for certain English linguistic features; closer attention to language contact theories and the linguistic evidence corrects some of its claims.