So far I have just one bikepacking race under my belt. There were many similarities to an Audax, with the main exception being the time element. I chose a mountainous off-road event, which was much slower and draining than a road equivalent. The chance to have a good rest each night (I was still underway by 4am, so not exactly touring!) and not be particularly pressured by time limits was a benefit in my opinion.
It took me 10 days to complete the route. During the first few days I did see other riders, and rode with some for short periods which was good. Later on I had whole days without seeing any other riders (although I could roughly judge where they were from their trackers). Indeed it got so remote in eastern Germany that I could spend most of the day without seeing anybody whatsoever.
For off-road events, the biggest difference is the difficulty in planning each day, as the surface and type of path can vary so much. For most long-distance Audaxes I have a plan that I tend to follow, deviating by exception. Off-road I took the first few days as they came, but later in the ride I was planning the following day the night before, and adjusting my start time to ensure I reached my intended destination 'not too late'. I guess through experience you pick up an understanding of what the average speed might be, but it'll fluctuate wildly.
With that experience under my belt I have a few similar rides next year. This time I will have more practice events before the big ones, to hone equipment and my routines. I'll also be slightly more in 'race' mode, so the sleep stops might reduce.