The serve, in squash is called an Hard Serve or Power Serve. But not many experienced players do it for the sole purpose that you are leaving yourself in a bad position for the return.
If you look at the picture above, You will see that you (obviously) need to bring your racket up in the air and then follow through with your swing, pushing your racket down. This does create a fast serve which makes it difficult for a novice player to return. But if you're playing an experienced player who knows how to return a fast paced ball, that player will most likely return the ball before you can recover from your "overhand" swing follow through. This is why most people serve underhand so that their racket will be in position when the opponent returns the ball.
To sum up pros and cons...
- If you are playing a novice player, this is a great serve because of the pace (speed) of the ball.
- It is good to use this serve if your opponent getting used to you using a normal underhand server.
- Any experienced player will be able to return this serve and most likely beat you because you have not recovered from the long follow through that this serve requires.