3 years later and their federation is still as beta as it is.
one can make a full fledged, completely working activitypub implementations from scratch for just 8 months or less, yet this one take that long and yet still imperfect
These are merely my personal opinion. Replies are welcome.
first of all, the delivery of a post from both parties is slow and terrible. it took them several minutes for a post to be actually received/delivered. Even though your post/replies gets delivered to Threads, They could not really reply to you. They could @mention you, But if your instance has nobody that follows that profile, It's just as that.
second, the mention one. Usually you had some kind of hyperlinks or similar when a fedi user genuinely mentions you, But when a threads.net user is doing so, It doesn't even look like an actual mention at all. Misskey might have handled it, But yet this one conflicts with the other fedi implementations. The problem is, If that's how it worked, Then probably threads mentioning some users might not send that user a notification let alone noticed at all.