Yeah I foud the harness completely changed the exercise from one where you had to stay upright to avoid losing the bar to one where you were pulled forwards by the weight. GetStrength are based in NZ and if you're involved in either powerlifting or weightlifting you've ordered from them and probably met some of them. Small country
Even using proper front squat technique and maintaining as vertical torso as possible there will be a slighly forward lean which increases the mobility demands compared to simply holding a loaded bar in the rack position.
One of the other major technical differences in the front squat that people run into is they try to pinch the scapula back as in a back squat. You need to shove the shoulder girdle forwards and up. Trying to squeeze your elbows together can be a good cue for bringing the shoulder girdle forwards and up and externally rotating the shoulders.
I'm not saying you should never use the harness, just that if you had trouble just holding a loaded barbell in the rack position without pain I'd be doing a serious evaluation of mobility and posture.