If you haven't found your own cards more valuable than cards other people made, you're making cards wrong, and could be remembering them MUCH easier.
So, the best cards you can make are cards that refer to concepts, internal to you, that are highly associated. What I try to do is encapsulate the statement that originally made me understand the idea, which is usually a metaphor or analogy for another concept I've deeply overlearned. Then I make a few other cards that elaborate on the concept to get away from the metaphorical link and into the specifics.
See also: https://www.supermemo.com/articles/20rules.htm . The rules that are related to what I describe here are 11-14. This is written by the godfather of spaced repetition.
I'm also going to lend my opinion: I agree entirely. If you don't find your own flashcards better than other people's flashcards, you aren't making the right connections or you aren't using mnemonics to aid learning. Personal ones work better a supermajority of the time (unless someone has a particularly clever mnemonic that resonates well with you)
Learning and remembering are very different processes.
I've had great success sharing cards and decks and haven't found my own cards any more valuable than the cards I've gotten from friends.