Forgetting about cost, which is a one time event, since CO2 itself is cheap, the only disadvantage to CO2 is complexity. The more things you add to a planted tank system the more complex it becomes. CO2 is one more thing to watch, monitor, maintain, adjust, etc. If you are a casual aquatic gardener, looking primarily to enjoy the appearance of your tank, go with low light and no CO2. That restricts your plant choices, but still can leave you with a beautiful aquarium.