The issue with shadowlings is that a single shadowling can convert the entire sec team to their side.
Traitors have mindslave implants, but those are expensive, require surgical removal of the mindshield first, etc.
Changelings can impersonate sec, but that requires one changeling per impersonated officer.
Shadowlings aren't stopped by mindshields, and a single shadowling can enthrall many officers.
Once the slings thrall sec, they've not only eliminated their main opposition, they've converted a group of people with excellent stun/disable weapons (ideal for capture/conversion) to their side.
At that point, the round balance is heavily in favor of the slings, and there's not much the crew can do to fight back beyond call an ERT.
If you want shadowling rounds to be more interesting... alter mindshields so that shadowling enthrall doesn't work against mindshielded targets.
Slings can still kill off sec. That should be possible. But subverting sec won't be possible anymore without using surgery to remove their mindshield first.
That should restore some balance to the game mode.