As the tin says, a Headless Client gamelogic even when set as playable doesn't show up in the playableUnits array.
Why is this relevant? If I wanted to setGroupOwner to a headlessClient with an unknown name by looking through playableUnits - such would be impossible.
Place Headless Client gamelogic and set to playable
diag_log playableUnits
playableUnits
Description:
Returns a list of playable units (occupied by both AI or players) in a multiplayer game. This does not include dead players awaiting respawn.
Note: Headless clients are "alive" as (alive h1) = true
You can already return hc with allmissionobjects. Adding it to playableunits might break many older scripts.
All virtual units won't appear in playableUnits afaik. This seems like a design decision and I think it's a good one personally.