Using a spaced repetition system for math has never worked well for me. It's useful for memorizing mathematical facts (for example, the mean/variance of various statistical distributions), but over time my intuition will fade, and I'm left parroting facts, having lost the understanding of why they are true.
In my experience, spaced repetition systems excel when I can create cards that take 10 seconds or less to answer. So they work really well for something like language learning. But it's difficult to reduce mathematical arguments into 10 second chunks．
What you say is generally true for mathematics, but it can help for the basics: Definitions of terms, etc. Basically, my goal was/is to memorize the facts one really should know without needing to look it up. For example, I've learned statistics at least thrice in my life, and beyond the very basics, the material doesn't stick. I think it's good for retaining a minimum amount of knowledge that you can use in the field any time you need it. Things like the recursive rule for the Gamma function, or that pairwise independence does not imply mutual independence (although a better flashcard would ask you to recall an actual counterexample), or what each probability distribution is useful for (I don't think I'd ever want to memorize the mean/variance, though).
I agree with you that overly relying on flashcards will take away the intuition, which is critical in fields like mathematics. At the same time, one can use flashcards in addition to other methods. It's not as if using one approach impacts your ability to learn via other approaches. Also, I think one can be clever in how they use them for mathematics. A lot of theorems rely on a certain key trick or two, so I think it makes sense to have flashcards asking what the trick is. I think flashcards are great for memorizing counterexamples, as well. Or heck, even regular examples. Depending on the branch of mathematics, knowing these is immensely helpful (e.g. in analysis).
Having said all that, I'm not using it for math presently.
> I agree with you that overly relying on flashcards will take away the intuition, which is critical in fields like mathematics.
Can't you just write a card that specifically asks you to give the intuition behind some concept?
E.g. "What's the intuitive interpretation of the gradient of a scalar field, \nabla \phi?" Answer: "\nabla \phi is a vector that points in the direction of greatest increase of \phi."
In my experience, spaced repetition systems excel when I can create cards that take 10 seconds or less to answer. So they work really well for something like language learning. But it's difficult to reduce mathematical arguments into 10 second chunks．