The Swiss ball leg curl is super hard and will expose your spaghetti hams. No matter how much hamstring curling you do in a machine, these will be a challenge.
The feedback this exercise gives is fantastic. If you can get yourself on video doing these, pay attention to your hips. It's important that your hips stay high through execution. This is how you get the hamstrings into a fully shortened position, and it'll help you find out if your hammies are super weak. If you can't keep the hips high, your hams are weak.
Do 6 sets and as many reps as possible each time. Hold the contracted position for 2 seconds before doing the eccentric (negative).