How secure is Stack Exchange compared to online forums such as Reddit or Quora? Better? Worse? How difficult is it to hack and how secure is user information?
There is something we can mention about the security of Stack Exchange; over the years, 19 users (so far) have discovered one or more security incidents. They are awarded a place in the Hall of Fame, which you can find here.
Whether those lists are complete, and how it compares to e.g. Quora, is impossible to tell unless you're deeply involved in both platforms, which hardly anyone is.