There have been several threads on here in the past on this subject.
The crucial things are that the harness stops the wearer falling out at all, rather than catching them when they have, and either the harness or the tether are quick-release. Industrial harnesses that are like putting overalls on are hopeless as they take too long to remove in an emergency.
I have seen people with ten feet of tether on the floor at their feet, and/or wearing the sort of thing found on building sites that are awkward to get out of.
I use a harness made by Irvin, it's a modified parachute harness with an adjustable length tether. The tether is attached to the harness with a three-ring release operated by a well-protected 'rip-cord' handle.