He's not saying it is not a good combo, just that watch your hydration. ALL fat burners/stimulants have a dehydrating effect. SO even when JUST taking a fat burner, you need to drink more water. Creatine ALSO has a dehydrating effect. SO combine the two, and you have to watch your hydration levels.
If you are someone who does not drink a lot of water, and cannot imagine drinking 1-2 gallons a day....then neither one is for you. Especially not together. They are perfectly safe to take together....you just have to stay hydrated.