I've had a couple of Kytheras.
You almost always need the slower metering screw, unless you're going to "fap dat trigger" put the lowest one in and then revel in having no feeding issues.
Complete PITA to get the alignment absolutely perfect
All of the screws need locktite
They really need at least a 300mm barrel or you'll be running really high pressure
Other than that, they are generally really quite sound engines, they are super air efficient, really quiet - mine you only used ot hear the click of the trigger mechanism, don't need a battery (bonus), the cocking wire only needs pulling once, if you de-air and re-air it'll recock on it's own.
I did a lot of deliberating between kythera and redline n7 milsim and don't feel bad having gone kythera.
This is all past tense though as I have sold both both my kythera builds now (but not because the kythera's didn't perform).