If you donate miles and it's not a charity that partners with your airline, you would pay transfer fees which often are more than the value of the miles.

OTOH, if doing the booking is complicated or the person wants to make a bunch of changes, then it could require a lot of your time--unless the miles are in an orphan account and you're willing to give the charity (temporary) control of the account (and even so, they might not be very good at using those miles).