I think the key point here is that the navigation setup could be different for different events. At the Safari rally I needed a roadbook and an odometer and that was it. I don't think GPS was allowed but it wouldn't have done you much good. I used a reasonably simple handlebar mount setup and did not find the weight on the bars to be much of an issue. The terrain consisted of loose sand roads and hard packed 2 tracks for the most part. In an event like Dakar with all the crap you have to carry a frame setup would be ideal given you have all the extra equipment on the bike.
My intuition nearly makes up for my lack of good judgement.
Just installed my new trolling motor