Isn’t the main difference just that forums are focused on longer discussions, and reddit/lemmy are focused on a constant stream of content?
I’d prefer forums for a lot of my interests, a well managed forum will contain long in depth discussion regarding important topics that the likes of lemmy/reddit/discord either don’t, or if they do, good luck finding it. If however you just want to visit it in the morning and see something different than you saw yesterday, yeah for just raw speed of content, forums suck.