A big drawback, and what killed many of the original forums, is spam management and maintenance. It's a very lucrative target to attack for two reasons. Popular public forums tend to be very central in pagerank, so if you manage to post comment spam, that's a big win. Forums also act as large databases of usernames and (maybe) hashed passwords, so if you manage to find a vulnerability in the software, you can sell th…
And the cost is lack of discoverability, basically forever. Maybe there's a way to layer on discoverability to discord, but I haven't seen one that compares to Google. Anyway, I repeat myself, I've already written up my thoughts on forums vs slack/discord: https://www.mooreds.com/wordpress/archives/3451
Those concerns are so painful that people are willing to use a tool (Discord) completely unsuited for what they want simply to not deal with them.
A centralized system like Reddit or Discord can amortize both of those over gazillions of users.