The disadvantage is the shorter range, but you make up for that in speed, recovery time, and less danger form missed hits.
Missed hits are only a real issue when you're going for the downward swing to the head. Any melee weapon is going to be more practical for pushing your enemies back. The rock hammer doesn't have that advantage.
The sledgehammer is unbalanced, which is what makes it such an ideal weapon for this purpose. Because the mass is at one end, it creates a much greater force. Hitting a body from the side won't just break bones, it will physically force the enemy to the ground. That was really the point I was trying to make with both of my melee weapons. If they don't kill the zombie outright, you at least want something that is going to knock them off balance or stop their advance.
It's difficult to hit something “wrong” with a baseball bat because it's more evenly balanced.
In this case, it's very easy. Hit the zombie anywhere other than the head or from behind and you've hit it wrong. A baseball bat doesn't deliver the sheer force required to knock people off balance with a hit to the body. That is its biggest failure. Actually knocking the zombie down requires you to get far closer than you want to be to your target and you have to be able to get behind them. Since a head shot may not kill them outright, you run the risk of having the zombie simply continue their advance upon you without giving you any sort of time advantage and your swing has put you within grabbing distance.
In terms of durability, aluminum bats have a tendency to bend easily when hitting heavier objects. They are designed to have some level of give so that they can create a sort of trampoline effect when hitting a baseball or softball. When hitting a larger, heavier object this caused the bat to crumple like the panels on a car. Once the bat is bent,the force it can deliver is severely reduced. A better choice for bludgeoning would be a wooden bat.
In addition, since the bat will rarely kill a zombie and you won't get the opportunity to pull out your gun, you're stuck putting all your stregnth into a frantic assault of swings until . The sledgehammer could be used for a single swing to the body, knocking the zombie back several feet and to the ground, giving you plenty of time to pull out a handgun for the finishing blow. You'll wear yourself out much faster with the bat.