One big problem. Envelopes for midi events (like CC1, CC11, etc) work fine within Sonar but DO NOT work when rendering to an audio file if you're rendering in real time. If you set up some midi envelopes and play them back... they do what you'd expect, automate whatever midi thing you're wanting to automate. Render out to an audio file using fast bounce, works like expected. Render out in real time... nothing will get automated. It'll be as if those envelopes aren't even there.
For lots of streaming samples where fast bounce is too fast for the samples to stream correctly, this is a huge problem. I've submitted a bug report several months back, got confirmation that this is indeed a bug but no fix in X3, X3a or X3b. I sent another email expressing disappointment that this hadn't been addressed, getting this huge bug fixed but haven't received a reply yet.