If you're working around bad posture with a cushion, then your upper body is not properly engaged to stabilise the weight, and your lower body is in a weaker position because the bar is further behind your center line (radius of the cushion), so you're compensating by leaning forward for the entirety of the movement.

You can use the cushion with good form, of course, but you probably aren't.

The issue is usually shoulder mobility, since you could just force yourself into a better position using the low bar technique otherwise. You can do front squats whilst you work on your shoulders, unless you had some injuries that will stop you from ever getting your mobility good.

In which case, I think you should use a Safety Squat Bar, instead of a regular bar with a cushion. This will allow you good form, regardless of your mobility situation.