Spaced repetition is a highly effective method for learning languages, particularly for vocabulary/character acquisition, but it is not the only method and particular implementation of it in the software, really matters.
It is the only method in that it is a generic term. Without spacing there will be too many words to review at once. Without repetition it will be hard for people to learn things in one try. Spaced reputation is required to scale. But do language learning apps need to scale to handle a lot of information? Technically no, but the incentives around making apps make it so that SRS will added and dominate the product.
I will agree with you that the implementation matters, but ultimately anki will cover one's needs for SRS and open source efforts should go towards improving anki.
