When I played MMOs Discord was often made mandatory by leaders for the voice features and setting up game time for a group. It was good for quick answers, but those answers were limited to whoever was on line at the moment.
Forums have their own major, even fossilizing problem, that being the lack of people who contribute to them. Every year, it seems less and less people bother to read...well anything, actually...so fast scrolling one liners become more popular. Still, Discord and other one liner venues, can give a user the sense that he is personally giving and receiving information with immediate feedback. That can be true when the user gets lucky enough to log in when multiple "good" users are on line at the same time. Unfortunately, the one liner style and the need for immediate feedback means that most of the time, hour for hour, Discord is going to give shallow or imperfect answers.
The major advantage Forums have is they bundle a question and the answers in one thread. Eventually the thread scrolls away, much like a Discord discussion does, but it does it much slower. That slowness is a major advantage for getting real answers to questions for the simple fact one must see a question to answer a question. Ironically, this same slowness may contribute to lower population counts on forums, versus one liner discussion venues.
While both Forums and Discord have search features, Forums have the advantage that questions are actually seen to be answered, while One Liner Venues have the whole "I" thing advantage. I typed and someone answered immediately. I matter.
In a world where most answers are equal, then Discord would probably excel over Forums. Since these forums make it clear that not all answers are even remotely equal, then Forums can be competitive in accuracy.
Still, many people want an answer now, not the better answer, later.