If you stand long enough at the education bus stop you see the same arguments chugging round and round. "Teachers underestimate the ability of unruly boys" (page 3). That's an opinion that has been picking up passengers since the early 1990s and is invariably part of a larger thesis that sees primary school as a matriarchy that suppresses boys.
Researchers have also claimed that teachers underestimate the intelligence of fat girls and short boys. So should we do away with teacher assessment and base all pupil judgements on external tests? That would hardly be wise given that many tests have also been accused of gender and cultural bias.
The sensible thing to do is continue to have a judicious mix of external tests and teacher assessment. Teachers may not be models of impartiality (who is?) but many are better judges of their pupils than any test.