I dun a wee bit o climin in my life an' my harness never failed me. I
did retire it when the stitichin showed some wear and made another. By
now, I probably climbed a couple miles of rope with no probs except
when I tried mechanical ascenders once. When I got to the top of the
pit, I realized the pin that held the cam in the shell of the ascender
had come loose, near gave me heart failure. Mechanicals also had the
habit of not gripping immediately which would put your heart in yur
mouth fer a fraction of a sec. Prusiks never failed me. OTOH, I have
friends who used commercial made stuff who are currently paralysed or
dead from falls although the gear was technically not at fault but I do
think they put so much faith in the expertise of others they failed to
give proper thought to technique.
I'd say my "gut feeling" is based on long term real experience.