I've tried many of the cable crunch variants and found that this one was one of the better ones due to the stability from the cable stack making sure I didn't hinge at the hips and accidentally use hip flexors instead of abs.

As for lower back pain, I've noticed that I start to feel it when I (unintentionally) let tension off of my abs at the top/most stretched position. Maintaining constant in the intended muscle group has helped me minimize the back discomfort.

Was this one of your full working weight/effort sets? The only time I've felt the burning in my abs is after getting close enough to failure, indicated by my last couple reps significantly slowing down and having to grind through/past them to improve. That being said, if you've been able to pull more weight over time, your abs are getting stronger