Just to recap how their Coilbeams function: They hit the primary target (like a Guardian) for the full listed damage, then do a second copy of damage, divided among everything else hit by the beam.
They fire every single second so should have little case of "wasted damage". Because the damage is divided, if there are AI units all sitting in the same place it might seem as if not much is going on, as they're not dying individually.
Since they split it evenly, they don't really need to actually hit many units to get their full damage. They could hit 5, 10, 20, so on, and it'd be the same damage total, unless the things hit don't have enough health to absorb the full power. Far as I know they are monsters in combat - we had to add a specific AI defense against being able to just rush to the AI Homeworld and assassinate it with the starting fleet, they were that strong.
I'd note unlike Classic, the Cities get larger over time and Mark up, the Flagship begins to upgrade (from Cruiser to Battleship to Dreadnought), bonus ships are added in for free, etc. So they might not seem as impressive at first, but they scale up to much higher values. Going from memory...it's possible to have a Dreadnought destroying 2 Dire Guardians every second, and Golems wouldn't last much longer. And you can get multiple Dreadnoughts. That's not considering Military Stations.
Here's some information on things:
https://wiki.arcengames.com/index.php?title=AI_War_2:_Fallen_Spire