Because I believe threats are far to player specific. The most common argument I see on guides is about threats. That's because everyone struggles with different characters for different reasons. A lot about guides is subjective, but I think threats are one of the most subjective aspects to guides. Characters that are threats are just characters I can't figure out. The characters that are the biggest threats to me are characters that have stuns. Why is that? That's because I suck at reflex block. Not everyone sucks at reflex block, so my threat list would be a joke to many people. I get wrecked by Koshka, but if you can reliably block her catnip frenzy she's fairly easy to counter. Krul is generally easy for me to beat because I understand that atlas pauldron destroys what he wants to accomplish, but a Krul player that is efficient with From Hell's Heart can cause me problems. A final example is Reim, I generally wreck people with Reim, but he's pretty easy for me to beat, because I understand that if you keep your distance and avoid damage you prevent his fortified health. A lot of players engage with Reim, but if as a team you refuse to focus him you can focus the others on his team first, or you take him out with a range player.

This of course is my opinion, but I don't find threats to be a reliable part of a guide. This is also similar in reverse. A character may be a great counter to another character, but if you aren't efficient with that character the counter isn't going to work. Threats and synergies are far more player specific than any other part of a guide.

Well this is why there are different builds for different people. There's no right answer, if you think it works then go for it.

For me it's not about "falloff". Honestly there's nothing to fall off. I'm purchasing Alternating Current primarily for the attack speed, the benefit of a shorter cooldown on Boomstick, and to allow me to apply the slow from Frostburn off of a basic attack. All the items work together in the build that I use Alternating Current. Sometimes it's not just about the damage, too often we get consumed with "how much damage".

Again about "falloff", it's not about that with Aftershock, it's about the efficiency. You say "rarely is this perfectly executed", to which I first applaud you for looking at items that aid in execution shortcomings, but in my experience if I fail to execute staying on the outskirts, if they get that close to me, I've already failed and the occasions are too rare in which Aftershock will save me in this situations.

You point out the cooldown factor, and how it scales late game. Well at overdrive buckshot bonaza has a cooldown of 5 seconds. If you overdrive it first you probably get it around the 7-8 minute mark? At level 8 depending on the order of your skill selection. At that point the 25% cooldown increase only changes that 5 seconds into 4 seconds, so the major benefit of Aftershock, outside of it's passive, becomes not all that much in the late game.

When we talk about synergy with Aftershock there are different tiers of synergy, and this is how I would characterize them.

Perfect synergy: Aftershock has perfect synergy with characters like Alpha, Ardan, or Fortress who have abilities which apply basic attack damage, thus instantly using Aftershock's passive.

Great synergy: Aftershock has great synergy with crystal powered melee fighters who use their abilities up close and are nearly immediately followed by a basic attack to take advantage of Aftershock's passive like Koshka whose abilities are almost always followed by a basic attack.

Good synergy: Aftershock has good synergy with characters like Lyra whose abilities and basic attacks can be made without a change of range. I place these characters in the "good" category because they still require targeting after the ability is cast so Aftershock's passive isn't a guarantee.

Poor Synergy: Aftershock has poor synergy with characters like Baron, Celeste, and Gwen who can cast their abilities from much greater ranges than their basic attacks, thus requiring a range change to take advantage of Aftershock's passive.

Let's make no mistake. The passive is the strength of Aftershock. The item doesn't excel in any other area statistically speaking. There are better items for cooldown, energy regen, and the CP isn't worth discussing considering I can replicate the 35 CP with a simple Eclipse Prism. Clockwork almost doubles the cooldown, and triples the energy regen, and with it's passive will surpass the CP easily. Stormcrown while not offering CP provides better cooldown and energy regen, and would make Gwen's basic attack somewhat viable in a crystal build. The point is there is really no reason to buy Aftershock other than the passive 15% of max health damage. Thus when we talk about how the item synergizes with Gwen, I just don't see it as viable.

I hope perhaps this explanation of how Aftershock synergizes with different characters helps you understand how I feel the item works with different characters. But perhaps you disagree with my rating method.