Maybe it's just me, but the obelisk seems to perform terribly in groups (or even when just mixed with other turret types) due to its long fire delay. The vast majority of its DPS gets wasted due the fact they will fire at biters that are already dead, with some even firing at biters that died up to two seconds ago. This is especially pronounced when they're used in large groups; during a biter charge every obelisk in range will focus fire on the unit in a charge and fire its shot at it, meaning no matter how many obelisks have, the entire group will only be able to kill one biter per firing cycle.

The issue is bad enough that I've downgraded a few dozen obelisks to vanilla laser turrets and I'm seeing significantly better killing speeds. The only niche I can see for the obelisk in its current form is to defend a large area against very tiny incursions, but even that conflicts with the fact the obelisk doesn't unlock until mid/later game so it's obsolete in that role by the time you get it.

As for a solution, it seems obvious: Make no delay between targeting and firing, much like vanilla laser turrets. This way if there's even a fraction of a second delay in other obelisks targeting the same biter as another, they will be able to switch to new targets and drastically reduce the wasted DPS. (You could even play the "charge up" sound during the cooldown between shots, rather than beforehand during the delay, so it will look mostly similar during periods of repeated firing).