The harness is useless,.. I used it twice & then discovered the job was easier without it!
Personally I'd try to avoid high pressure on gutters as there is a slim chance of doing damage. I use an x-jet to spray chems on the outside of gutters to clean them. The jet is barely more powerful than a garden hose.
Cleaning out the inside of the gutters can be a pain. Use a high pressure nozzle and it'll blow the contents of the gutter everywhere and also make the lance very hard to control,.. but if you use a low pressure nozzle it just washes the dirt into the downpipe & can cause nasty blockages. I still often resort to ladders for this but, but plan on a hybrid system with a low pressure nozzle to loosen the dirt & a smaller than normal wet vac to remove it.