you already said that, but like, what does it mean

What forum/s are we talking about? What discords are we talking about? What kind of botting are we talking about? Are the forums actively moderated? What's the moderation experience like? Etc...

The big problem here is that discord is the only thing between you and managing your server, either they do it right, or they don't (which is what they usually do) With a forum, you are the only person between the two, but you have full control over it You want to set up a basic entry gate to prevent bot accounts from sneaking through? Trivial. You want to set up message rate limiting? Trivial, you want to set up some clever detection and sequestering mechanism? Trivial.

With discord you're either using bots, which suck, or you have to pay for, or you just pretend that it isn't a problem and let humans deal with it.