Most people don't use flash cards at all. So if their experience with them is terrible then why would they keep doing it? Also effort can be fun. Working out is effortful and most people don't do it. So, they should go to the gym and try to have a good time. If they aren't, maybe the advice to do some 5x5 program or Jocko whatever workout is not good for them. It's just a heuristic to stay motivated. But yeah, my friend just finished med school and memorized like 30k extremely complicated cards. He did not "have fun".