I agree with Jacquelyn above, it depends on their end goal. If someone is looking for an intense run that works extra leg muscles (including lots of calves, ankles and foot muscles), adding sand runs to their workout routine is definitely a good challenge. Like any new activity, it should be added in by slowly increasing frequency, intensity, and time.
However, if someone is training for a running race (I.e., 5k, 10k, half-marathon, etc), most of those races occur on hard surface. In this case, the runner needs to be trained to run on concrete or whatever the surface is for the race course. If the runner has only trained on sand, grass or even treadmills, their joints and muscles will not be prepared for the harder impact. Although they will have strong legs and will have built up endurance, it will most likely cause injury to one or many muscles and joints. That being said, adding in a training run on sand, trails, hills and/or grass once a week or even every other week is a good way to recruit more muscle involvement in their running pattern and break up the monotony of run training.
Love the sand. Like anything else you need to adapt to this challenging work. Start slow and listen to your body. It’s really fun to wear a heart rate monitor and appreciate the great stability and cardio work. Concrete is to be avoided if possible. If you must, use a smaller stride with a soft landing style of running. Have fun in the sand if you can!
First I’d like to ask, “better for what?” It’s always a good idea to find out what the ultimate goal is before deciding on a course of action.
That said, I agree with Karin. Running on sand is definitely more challenging from both an intensity and stability perspective. But that isn’t necessarily going to be appropriate for everyone. Also, if someone is training to run a race which will be on concrete, running on sand may not be the best way to train for that goal.