For forums you can look at them for a minute or less and figure it out. It's same with HN and sub-reddits. They are information dense and don't require signing up or customization to figure out if you can curate them into something you
might like.
Compare that to mastodon, step 1 is pick a server https://joinmastodon.org/servers
Very few are actually topic focused. Even the ones with themes have a lot of deviation. Even picking the popular safe choice - https://mastodon.social/ , I can't tell if I would ever like it. I don't like what I normally see but considering I can see 2 posts at a time and a significate portion are animals or other topics I wouldn't visit a forum about, it doesn't feel like I would.
And the animal thing is common across most of the servers. I understand it, I have dogs. But it's a side effect of the medium not having a coherent focus. It feels like I'd have to spend so much time to turn it into something that I'd like that I'd be better off staying with forums.
Mastodon seem great if you want to follow people and be social which is kind of the point of social media. I want to follow areas of interest, not people.