As with any hip thrusting variation, a good rule is that you should always be able to squeeze and hold the contraction at the top for a one-second pause. If you can’t, the weight is too heavy. Pausing the reps at the top also helps to ensure that you’re using your glutes and not just hyperextending from the lumbar spine.
If you’re feeling this exercise in your back, you’re not doing it right and you probably need to lighten the load or slow down the movement (or both) and focus on doing it correctly. This unilateral version is more back-friendly than the bilateral version.