1. 1.
    External consensus mechanism: The sidechain may have its own consensus mechanism and the attack vectors associated with this needs to be reviewed carefully.
  2. 2.
    Deposit and Withdrawal delays: When an entity wants to withdraw funds, they need to wait for the next commitment / merge mined block on Bitcoin, and subsequent confirmations on Bitcoin in order to withdraw their funds. This is usually slow since entities wait for a certain period of time before confirming a transaction on Bitcoin.
  3. 3.
    Not time tested: Sidechains are a relatively new idea, and their security has not been as time tested as Bitcoin. Sidechains however, have not suffered attacks like Stellar.