On a forum, the chance of your question being totally ignored is much, much higher. Some do have some staffs that seem to be obligated to reply, and they will just.. copy and paste some templates.
On Discord, even the devs and staffs are not always there to answer questions, there are often enough other users that can help you, and they are willing to discuss with you if details are not clear (as soon as you're polite). Even though they don't always solve the problem, you can tell someone actually looked into it. And all these happen in real-time, without at best half day delay between each exchange (it helps that Discord is hella popular so lots of people are online all the time, and the chance to notice your message on a server they're in is much higher. Can't say the same for any random forum.)
I still prefer GitHub issues, but after that, Discord. Forums (or the communities it normally forms) really don't cut it.