The benefit of B2BUA with regard to helping NAT traversal is it does not require any modifications to the client to make it go through NATs. And since basically it is a relay, it should be able to traverse symmetric NAT successfully. However, since it is a relay, the usual relaying drawbacks apply, namely the bandwidth and latency issue. More over, since a B2BUA acts as user agent in either call-legs it may also introduce serious issues with end-to-end SIP signaling.