The main disadvantages of owning a Sheltie include excessive barking due to their herding background, which can be troublesome in close living spaces. They may exhibit herding instincts like nipping at heels and chasing moving objects, require regular and frequent grooming to manage their thick double coat, and can be prone to anxiety and separation stress when left alone. Additionally, Shelties need consistent mental and physical stimulation, and they tend to be clingy, wanting to stay close to their owners.