"If anyone is likely to have accurate insights/judgment into the impact of particular techniques on a language learner, let us hope it is language teachers about their own past learning of other languages."
Shouldn't teachers have some special insights into the learning process derived from their own experience studying a language?
In fact, this experience can be very harmful.
Some students are academically inclined. They read something, they remember it. The teacher says something, they remember it. They study the books in the library and they do everything correctly in class. Sometimes they even sit in the front of the class.
These people often become teachers.
They are remarkable people. We cannot criticise them, only admire them. As students they can smilingly sit through the most boring lectures and actually pull some jewels of important knowledge out of the verbage. They can study the most complicated texts, decipher them as well as any CIA analyst and file away the data into different parts of their computer-like brain and retrieve it later when the teacher demands and to his pleasure. (Indeed, such students make teachers feel like gods.)
The problem is the other 80% of the class, what to do with them?
Some of them have blank looks on their faces. Some of them just don't get it. Some of them are bored to death. In extreme cases, some of them have slipped into a teacher induced coma on top of their desks.
When good students become teachers and then draw on their learning experience when dealing with their students they can make a big mistake. They can be tempted to believe that their students are like them and can learn like they did.