The point is debatable, but essentially - no.
Romulan ones, yesā¦ Klingon, no.
Nope. Just take the 1 point difficulty increase for non-lethal use of lethal weapons.
The Threat cost of lethal attacks is in line with Starfleet values of non-violent conflict resolution, values that they do not necessarily share with the kind of enemies willing to zap 'em! However, just because this thematically it fits the rules doesnāt mean itās the reason behind those rules, so it should not be inferred that the rules do not apply to Klingon or Cardassian enemies.
Iām fine with that.
If I ever run into a problem because the enemies canāt kill off the PCs because they have run out of Threat then Iāll see what the situation allows for. Maybe the enemies call off their attack and retreat, or allow for a cease fire, or maybe through a tremendous amount of luck all their remaining attacks are non-lethal without them noticingā¦
coup de grace should be D0ā¦ so they fail one, by being sloppy (doing non-lethal), but use it to build momentum to go around to the others with lethal attacksā¦
The difference ceases to matter if the PCs are on the losing side.