It's not terribly hard to find examples of second language acquisition researchers expressing skepticism about flashcarding, though. I don't necessarily find all of it universally convincing (if I did, I wouldn't have 99 reviews left to do today in my 中文 deck), but I do have to admit that a lot of the evidence people claim in support of using spaced repetition systems for language learning has the look of pseudoscience. It largely consists of taking some very limited, tightly focused lab studies, and massively over-extrapolating from them.

But I wouldn't want to over-extrapolate from the studies that claim flashcarding doesn't help, either. The protocols often focus on a very minimalist implementation of flashcarding that's easy to study. Or the protocol is really designed to test some theory about how memory works and not some theory about study strategy. (This second example describes basically all of Ebbinghaus's work, by the way.) One could easily argue, for example, that using a flashcarding application to rote memorize a pre-made deck of words and phrases that somebody else chose is a fundamentally different practice from sentence mining your input materials.