The danger in using a climbing harness is the reduction of blood circulation in the lower limbs, especially if your legs are immobile and partly 'strangled' when working from a harness from long periods. The potential is, when when you later move and the partly devitalized blood begins to recirculate the altered blood changes the biochemistry of your circulation system and can produce an acute 'metabolic shock' that has the potential to set off quite severe cardiac dysrythmias etc. If you use a harness and especially if it 'pinches' your leg arteries and major veins .... you HAVE to keep moving your legs instead of 'hanging there' for long periods of time.
Such 'climber strangulation' is obviously quite very rare, but you should be aware. Lots of such 'injuries' becoming evident to the 'mountain rescue' community. Possible solution: BIG WIDE leg straps on climbing harnesses.