Somoan, as far as the harness is concerned, for tuna it would depend on the size of the fish, I guess above 50lb. is about the point where the harness goes on.
Shark fishing is a different story, if I know I have a shark on, but, don't get a visual on it to size it up, the harness is on. Big Mako's can start off feeling like small fish, the other way around, sometines you think it's a Blue Shark only to find out it's a Mako, etc., etc., etc.
You should not really be using your arms when fighting a fish with stand up gear except to guide line back on the reel, and crank. You primarily pump the fish with your legs and without a harness your arms arm simply connecting your body to the rod so it does not go overboard. Wearing a harness is a VERY EASY way to learn the proper technique. When you are strapped in, you can, (assuming you have a right handed reel) put your left hand on TOP of the reel to guide the line with your fingers, and crank with your right. You can fight the fish without touching the rod, without using your arms. It's all in the legs, and the harness will teach you how to use them properly if you try what I say.
Just my $.02
MakoMatt