I'm using Nero 126.96.36.199.
I never see any message indicating buffer underrun has been enabled, but several times when I've burned a DVD movie, I've done something which hit the hard disk enough to cause an underrun. Nero simply continued, after it refilled the buffer, and the DVD played (and scanned) fine. I've had at least 3 DVD movies with 2 or 3 underrun events, none of which had any problem.
My view is that the seamless link for DVD burning requires no application level intervention, which may be required for other forms of underrun protection or burning.