Sit-ups can do long term damage (as noted previously) unless you place your feet against a wall/door and keep roughly a 90-degree bend at the knee & hips. IMO sit-ups/crunches provide too little gain for the time spent.
I've done a modification for a several decades that builds far more muscle mass. It's done in a sitting position on a cable machine which has a station with a cable position at shoulder height. You just do a crunch with weight. I use 80 to 90 lbs.
If you work out at a gym, they'll have a different type of machine to do the same thing.
When I did wilderness canoeing, I'd do three sets of 12 twice a week. Now with a lot of cycling, I only do two sets about every 8 or 9 days. Core strength, which this exercise and push-ups are key components for me, is very important for all activities.
Al