Making flash cards is in the low gain category, and that is very popular. Disciplined testing using those cards wasn't nearly as common from what I saw, people just made them and then forgot about them. Or just read through them without putting in effort trying to answer each one without looking at the answer. Doing those properly requires effort.
People do it all the time because it's fun and you get the initial kick. Like planning a budget, habits, hobby, exercise routine or spend a week collecting courses. Then the daily grind starts and you give up after a few days.
I, personally, just am reading a novel series Japanese, 8 books in. While reading, I have highlighted all the words/parts I didn't know in order to export them and make flashcards out of them. I haven't still started going through them, but the thought of doing so feels laborious, so procrastinating is bound to happen.
Finnish is a second language to me and what I've found better than flash cards is to just write down words I don't know as I'm reading. Initially this can seem terribkt slow because each book with have its own vocabularly set. As I continue if I read a word I don't know zi scan the list briefly. If it isn't there or I fail to see it I write it down again.
If you do not have time to study... I had to do long commutes while working part-time to get my CS degree.
Sometimes I was left with only 1 day of study for an exam and this was the best method for me.
