Are there any relative (dis)advantages to mounting the folding front sight on a free-float rail versus the gas block on the barrel?

I like the rail mounted BUIS for aesthetics but wasn't sure of any accuracy issues.

Well for one its not attached to the barrel and the foreend can flex.  For another its bolted on and that is a weak point.  

That said I prefer rail mounted BUIS.  The BU means BACK UP... as in NOT primary.  I could care less if I get 1 MOA vs 2 MOA groups at 25 yards using irons in an emergency.  I will probably never need to use the irons anyway and will only do so for fun every once in a while.