Okay, let me just note something really important: Those 4 dice vs -6? That's IF they even know where you are! Which requires secondary sight (drones, ARO, stuff like that) or them obviously seeing your gun sticking out, or hearing where you are, etc. So if you dive into a corridor and they fire through the wall, they won't be able to hit you just like that.
I had players hack marks, then use Trace Icon + AROs to let everyone see where the enemies were. That was a perfect appliance of the -6 vs 4 dice. But without it, spraying the building likely wouldn't have hit a single person. I had a player use their vision gear to give the sniper sight of the enemies, who used that to reason where he had to fire. Shot a bug spirit through two walls. That's lovely tactical combat. Using Periscopes to be behind Full Cover yet still being able to see the enemy's attacks coming and being able to fire at them? That's tactical combat. It's already all in the system.
By the way, Defender Unaware makes clear not being able to see the attack counts. So yes, normally in combat it doesn't matter that the enemy comes from behind or whatever. But if it's a sniper, or your sightline is completely blocked, you fail. Basically, in combat you'd assume 360-degrees vision for your players, so only things they still cannot see can catch them unaware. Full Cover without periscope means unaware, while 99% cover (or 100% cover with periscope) means not unaware so +4 dice for you. If you're behind full cover with the periscope, the enemy takes a -6 and wants to use a grenade, if you're behind 99% cover a good shot can still hit you just fine, or flush you with tactical combat. So again, already in the system.
So the question is: Is it bad that a bunch of poor shots that spray a lot of ammo in the timespan of 1 second still miss the super-reactive dodge monster that's going all movie-crazy on them? I don't think so myself. And a single lucky shot still kills.