What's missing from this discussion is a take that isn't entirely subjective.
What scenarios are there where scylla cannot be stopped, or is too effective for cost even if killed?
Saying "it's not tactical" means nothing and the only information that can be inferred from such a statement is "I don't like it". Fair, but there will always be things in the game that some players don't like and that won't change just for that reason.
Scylla is often misued. It can't even kill factories for cost and yet players often use it for that purpose.
There are two scenarios where scylla shots really shine: taking out unprotected eco (which tends to explode and destroy surroundings), and shield balls/unit clumps, which have other counters and are hard to shoot at when moving.
Once in a blue moon you can target reef/shogun when the enemy is not paying attention and destroy those, but if players keep ther units moving, it's hard to predict shots and even harder to make cost.
Silo has significant advantage over scylla because it can make different kinds of missiles (mainly shockly, but also quake and inferno when the situation calls for them) and more imporantantly, it can be assisted so building a missile doesn't take as long. Eos also has more range than tac nuke if memory serves.
I agree with
tivoris. Scylla makes it so you can't entirely ingore water on maps that have no metal in it. Water can still be used to sneak attack, and all you really need is an owl or a few well places urchins to screen for a potential strike. It really doesn't take much to detect or even deter scylla, and a failure to do so and not protect eco should result in catastrophic losses.
You can very quickly take out scylla with either sea units or ravens and if the enemy is already countering those to support the scylla, odds are they could also have just made a silo instead and the result would be the same.
I think there were something like one, maybe two games I in which I was able to send a naked scylla right to the enemy front and take out eco from plain sight, which is the one case where scylla does a lot of damage for cost without the requirment of building a silo in vision of the enemy. It's excessively rare simply because things that enter sight usually either dominate or die.
The game needs to end somehow. I'd be far more in favor of nerfing artemis than anything else.