So far, the only negative side effect I have seen of #threads supporting ActivityPub has been the whining and demands for preemptive censorship (from people who have more in common with the book burners than they realize).
I haven't seen a single objectionable post from Threads.
My hope is that I'll be able to delete my Threads account and follow those I care about from my Mastodon account. Mastodon gives me some decent tools for controlling what I see (lists, filters, blocks) and doesn't push me into an unwanted algorithmic feed. I have multiple UI options from both apps and the web.
I understand that any time a social space is opened up to new participants, it is disruptive. But I don't really care. If you want a private discussion space with a culture that you control, you should be using a closed forum. A federated open social network is not the best tool for all purposes.