Not necessarily — if you do them without yanking at your neck, avoid using momentum, and actually lift from your core instead of your chest or hip flexors, crunches and sit-ups probably won’t hurt you. However, they’re also far from an effective use of your time and energy. Remember that you get good at what you train for — so if your goal is to get really good at torso flexion for high reps, then you’ll likely accomplish that goal with high rep crunches and sit-ups.
But that’s probably not your goal. If your goal is to forge a stronger core that can help you increase your lifting load and decrease low back pain, you’ll want to avoid spending time on crunches and sit-ups, and opt instead for multi-purpose core exercises like weighted one-sided carries, ab rollouts, and even planks.