Disadvantages of using sim card from old iPhone?
In terms of using the phone day to day, there's no disadvantages with re-using the SIM card from the old phone.

The disadvantage lies primarily with the fact that if you had requested a new SIM card from your provider, you would have received one that could be running newer software (inside the SIM card). This could be relevant if you were the target of a specific hacking attempt, as the SIM card software is rarely, if at all, updated by providers after it is sent out.