Complex sentence: Nevertheless, testimonials from adult learners who have read her books are widespread--a recurring theme is that the very same Chopin Études on which she based so much of her teaching had been a stumbling-block until her methods were applied.
Split into simpler sentences: Nevertheless, testimonials from adult learners who have read her books are widespread. A recurring theme is that the very same Chopin Études on which she based so much of her teaching had been a stumbling-block until her methods were applied.