There's no algorithmic advertising, no universal tracking, and your instance's mod team is known to you instead of a faceless automation farm.
> using Discourse for a private group and Twitter/Facebook for public groups
Even if what freeone3000 said about Mastodon is also all true about Discourse (I know nothing about Discourse, but I'll take your word for it), it's not true about Twitter/Facebook. And since Mastodon is aimed more at the public group use case, Mastodon is a useful replacement of Twitter/Facebook even by your own logic.
I saw pornography of very dubious legality within seconds of opening a public Mastodon server during my first time checking out the fediverse. It was enough to convince me never to use Mastodon again.
I beg to differ. There is no technological hurdle to prevent this. Only privacy-through-obscurity, and who knows how long that will last?
The only missing part is using it to build profiles and join profiles. It may already exist, but I haven't heard anyone doing it... yet.
The scraping services also won't know your email address unless you post it publicly or they figure it out some other way, so they'll find it much harder to associate the data they scrape with your other online identities, too. This also makes the data less valuable to advertisers, so I imagine there's less incentive to scrape it.