Advantages: 

• Lots of freedom for movement
• Great for exercise and strengthening

Disadvantages:

• Very little support for the hip and back
• No arm support, except on the work surface—proper arm support depends on the work surface height
• Height adjustment is difficult-based on the size of the ball 
• It is also difficult to move the ball front-back or side-side
• Ultimately, much greater effort is needed to maintain good posture more than a few minutes compared to a properly adjusted ergonomic chair