I heard often about activityPub being challenging to implement.

Now I know part of this is because, if you are building on activityPub, you want interop with existing platforms such as mastodon, and they do their own thing.

But ignoring that aspect, what is so hard about activityPub? What could have been done better?

I am a software developer, so feel free to use software dev concepts and terms when explaining. Thanks!