Broken bones at best, pinned under a dead horse at worst. Those guys shouldn't be able to just casually get back on their horses. Second thing I'd like to point out is that falling from horse is some dangerous shit, especially if the horse falls over too.
Easiest solution would be covered holes in the sand to catch the horses' hooves. That said, it should of been done in a visually obvious enough way as to require no explanation.
I think that if a small military is going against a much larger and more advanced force then they should use guerrilla tactics because they would need every single advantage they could get.