I can think of a few things regarding the phasers at AR-558, though without these ideas, it's a clear continuity fault.
Idea 1: In sustained ground combat, enemies deploy some sort of dampening field, jammer, or area shield that only more targeted fire can penetrate successfully. (Most likely a jammer or dampening field since we never saw shield effects on a large scale in ground combat, except for Borg personal shields.)
Idea 2: While you can get away with a few bursts like that on a handheld phaser, more than a few wide-area or extreme-intensity shots like that will either a) fry the power cell, b) make the weapon explode, or c) otherwise take your weapon out of service and make you unable to employ the sustained fire necessary when dealing with multiple waves of baddies.
This should've been explained if it was the intention, though.
Idea 1: In sustained ground combat, enemies deploy some sort of dampening field, jammer, or area shield that only more targeted fire can penetrate successfully. (Most likely a jammer or dampening field since we never saw shield effects on a large scale in ground combat, except for Borg personal shields.)
Idea 2: While you can get away with a few bursts like that on a handheld phaser, more than a few wide-area or extreme-intensity shots like that will either a) fry the power cell, b) make the weapon explode, or c) otherwise take your weapon out of service and make you unable to employ the sustained fire necessary when dealing with multiple waves of baddies.
This should've been explained if it was the intention, though.