What are the disadvantages of a defined benefit plan?

You have no say in how the money is invested. Moreover, you can't choose to invest more in the plan. If you want to save more for retirement, you will need to do it elsewhere, such as through an IRA or a 401(k) - if you have one.

And while it's nice to know exactly what you will receive as your payout, some defined benefit plans do not adjust your future payouts to keep pace with inflation.