It's the A7750 automatic chronograph movement with seconds at 6 o'clock that's potentially unreliable.
This is because the movement is made in the seconds @ 9 position configuration. To transfer the seconds to an alternative configuration (ie. sec@12, sec@3 or sec@6) requires extra gears. Not all of these gears, or rather, not all the required gears are jeweled (that's why you'll hear people getting a 'jewel mod' for their reps), and as such create extra friction and wear and tear on those particular gears, coupled with at times, lackluster quality control in the assembly of these movements, you can see why that configuration on the A7750 can be at times unreliable.
Quartz and 21J don't apply, as there are movements that are made with the sec@6 configuration.