There are pros and cons of training horses in deep sand. Researchers investigated how footing affects equine athletes. While muscles adapt to the sand environments their findings show endurance horses training and competing in deep sand are at risk of pelvic fractures or bone stress injuries.
Source: EquiManagement, May 1, 2023. Link.
An endurance horse that works in deep sand should be evaluated for pelvic injury if there is lameness, poor performance, and/or unexplainable development of irregular gait patterns.