Asterisk as a B2BUA

What is/is not missing in Asterisk in order for it to function as a full SIP B2BUA, along these lines?: … User+Agent

Specifically, if I wanted to join together two other SIP Proxies with SIP as a B2BUA what would be stopping me from having full and appropriate 3rd party call control?

As far as i can see from a quick look at that article, that’s what Asterisk does anyway!

I am not so sure that it does it ‘properly’, as I am coming across things like this:

Trying to get to the bottom of exactly what Asterisk is missing to be a full-blown B2BUA, if anything?

May anyone shed some light on this?