I'd think a weapon like this would potentially add to the game. For one, there is no weapon that outright stuns as it's function. If enough damage is done in a short enough period of time, enemies are temporarily stunned, but that's handled differently. A weapon like this would be able to stun regardless. It'd also add some strategy. Berserkers could have to choose between killing outright, or incapacitating, specimens. Stunning would make him a bit more of a team player, making it easier for others to finish off tougher enemies. The long windup/recovery time would also add to the strategy, since it'd be a calculated risk. While the ax has a slow rate of attack, it doesn't really leave you vulnerable if you whiff. This could.