Imagine Ethereal getting played by Arachas Swarm or Gerni (or any MO deck, as all of them have the potential to spawn 4 drones in a row with the 4p card). On the turn they play Ethereal, there are two Ethereals. If you kill or lock one the other one will keep doing its job. We can make it harder for them by locking the one on the right so they will have to play between them for the first one to keep doing the job, but the locked one can be consumed so that the first one keeps doing its job.
Unless you kill two of them in one turn, it is actually an unstoppable engine! Saul can be reset or tall punished and this needs to be row punished. I would say this is a bigger nuisance than Saul as Saul can't be played in turn one and he needs some setup. I would definitely try this with Arachas Swarm and Gerni (and then get fked by Hefty and Amnesy and I would realise.. 'ooh we are playing Gwent, so lets not play any engines and move to some other swarm decks or NG' and give up on that)
My bad, it seems i didnt think this through, i was thinkiing you could simply lock or do 4pt removal and "stop the train", but its not that simple as you pointed it out.
Now i see why its "unstoppable" - because there are very few ways to deal with 2 engines in the same turn, and if you deal with one by removing or locking, there will always be 2 or more left so that wont help at all.
Still, with arachas you only have 5 tokens, so im guessing a Gerni deck will be more appropriate for it.
And i thought the 'ethereal propagation' could be geometric (1-2-4-8) but that is not the case since even if you have many, their effect will always only affect one unit.
But it gave me an ideia for a deck - all you need is an ethereal and a strays of spalla with gerni leader, then fill both rows with ethereals and spend the remaining cards on specials to boost and damage, since those dont take up board slots