I think this is a comparison of waist to boardshort seat harness. Not necessarily hardshell only. Ive been kiting 17 years, old(er) dude- 59 yrs. In fairly good shape- but BMI 25.
Waist harness Plus:
freedom of hip movement
ability to go into deep crouch
RE is quite comfortable and unobtrusive
Waist harness Negative:
when learning to foil you spend a lot of time in the water- way more than learning regular TT or Surfboard kiting- rides up and restricts breathing
Pull is higher up and when you have learned to lean and charge upwind, the high pull is tiring
Higher pull is more difficult to handle during power spikes during manövers.
You cannot wear a floatation vest ( important for me)
Hardshell harness takes a lot of space when travelling- its heavier.
So the boardshorts harness has all the positives of the negatives above. but to reitierate
negative:
It restricts the crouch and hips but much much less than for example a Dakine Fusion- which i tried and sold.The board shorts have 10% of that problem. its not an issue.
positive
its packs really small and is light
feels like i dont have anything on really
Easy to breath
can wear a floatation vest
protects my wetsuit from foil attack
lower point of pull helps a lot in hydrofoiling.
edit: the Dakine 2018 rope spreader bar has good and convenient attachment- its easy to set and release- it does not glide out of adjustment. The 2017 version was a POShite and dangerous- so i had buy the 2018, and didnt get any refund/discount from Dakine...When i had a problem with RE- they sent me a new one via retail from USA to Sweden after i sent photos to them.
If i summarize there are 3 main reasons: i can wear a floatation vest, the pull is lower, it is more comfortable.