It's intended to belay you in an emergency, allow you a brief rest or to gather your wits and equipment -- but not to hang for an extened period while working on some intricate problem. As Rich point's out, you need to be cognizant of that.

But I think you need to be aware of that in a bosuns chair too. For the first 18 years I had our well made -- but conventional chair, I basically did nothing to adjust it's back straps. I was safe, but not comfortable.

Then one of North Sails' senior people borrowed it to go up the rig. He let-out the ties so that you could get more support on the upper thighs by moving your ass back in the chair -- he asked me, "Isn't this a bit unfortable to sit it?".... IT's a lot more useful and easy to work at the masthead with it now.

SO, regardless of whether you use a chair or a harness -- you really need to adjust it properly. To "tight" can be as bad as too "loose"! (Rght, Lautrec?)