Oh but Jim, you say, I still don't see how federating with Threads is dangerous! Who cares if some Meta content is in my feed? I can always block it!
Well, yes, you can, but not everyone will.
In fact, most people won't.
In fact, a lot of people will be very excited and happy to connect with people they know on Threads!
And many Masto Admins are falling into the trap of thinking that's a good thing.