Well firing the lances at the same time simply draws too much energy from the ship - thank god it affects the engines instead of say the life support... The engines do not overheat, they just simply stop working due to the lack of energy being supplied to them.

Firing all the lances at the same time is more a last resort action than a viable and often used strategy. Besides the Fenris does have the rail guns for at least some protection when the engines collect energy.