Crunches and sit-ups can cause injuries to your spine and neck. You can end up with back pain or exacerbate an already injured back.
Crunches are not particularly effective at getting rid of belly fat, despite the infomercials selling you easy sit-up solutions. They do work great on trim, fat-free bellies!
Instead, do planks (properly) and workout your core without crunching your back bones and muscles. You need to strengthen core muscles to protect your back. Planks are great for this.