Ah yes, thanks for the correction.

Meh, this was fairly predictable what with how fast it got released after the update and how little FW generally care about rules.

To be honest the thing that irks me the most is the storm eagle, and not at all for the rules. Given that FW is essentially a 'fluffy' model making company, the storm eagle just seems to go agaiinst so much pre-existing fluff. Iirc, the storm eagle was supposed to be the main carrier for the marine legions up to the heresy (with the thunderhawk being the front line assault carrier). Surely that would make it enormous, no? Not that it really matters or anything though.

Oh, just in reply to "Many of them will simply fold if faced with something they're not prepared to fight" Yes, you point out that the imperial forces fold to the nids (or at least can't killl them) but that's a case of cyclic imbalance, and the nid army would be as expensive to assembe as either of the others (or at least on the same scale).
A one-trick pony is an army that relies on a gimmick to win, such as a special movement combination or ability coincidence. Weight of fire, weight of bodies and fielding semi-impeervious units aren't gimmick strategies, they're strategies that work stunningly well against all but similarly powerful armies or armies designed specifically to take that one strategy to pieces.

But that's not the point. The point is that any one of those spam armies posted above could pretty much flatten a non-spammed army at the same points just by sheer ridiculousness. At this point, someone with more money than sense can come along and buy such an army, and completely outmatch people far more skillled than themselves purely by virtue of having had bags of money. That isn't a good direction for competition in a hobby to go. (Especially not such a monopolistic hobby run by a company with an already duious pricing policy.)

I wouldn't generally be bothered by a situation like this in a game. As you point out, there are loads of incredibly powerful lists and they all have hard counters, this would make for a fairly balanced, if boring, competitive environment with a constantly shifting metagame in the top tiers.

Unfortunately, the world isn't perfect, and the strongest arguament against 2 FOCs I think is that such a shifting metagame, whilst acceptable in a relatively cheap game like mtg, would be ridiculous for competitive play in 40k, because the cost of remaining competitive would likely be measured in thousands of your chosen currency per year (not to mention painting time).

Also, the competitive player base almost certainly isn't big enough to create the momentum required for a central meta-cycle, and this would just prove a headache for absolutely everyone.

You've got to remember that GW's playtesters probably all use beautifully painted, fluffy armies, and I doubt they'd give a thought to anyone puttiing 6 of exactly the same unit in a list.

Not to mention that they've made it quite clear several times now that they don't give a rat's arse about competitive play aside from wishing it would go away for some reason,

Unless it was faq'd, the vindicator comes as standard with a stormbolter and the ability to purchase another or a HK missile. That gives the cannon a bit more survivability.

Scarabs aren't Troops though, and neither are Spyders. The usefulness of the mechanic in Tyranids comes from the scoring, staying power it provides (though the termagants don't really add much in that department that the tervigon doesn't already posess unless you're against an army that doesn't have enough AI power).

The other use in tyranids is as a cover screen for mid-sized beasties. Necrons also have very little use for this in the army type suggested, because it isn't using much firepower, cover screens get in the way of other units for assault and the scarabs want to get into assault themselves, which gives theme mutually exclusive movement priorities to those of a cover wall. (Also, you can't ignore the cover they give the enemy, like a hive guard.)

The problem is that you seem to see growing an army beyond its original size as an end in itself. It isn't. If it was, we'd all be playing five-tervigon armies (note: those are really, really bad). Adding more bases to a scarab swarm tends to do very little as far as my experience goes.

In general, whenever you fire effectively at a scarab swarm, you're probably going to overkill anyway, so the extra bases may well die to the same shooting. If they don't, one or two bases of scarabs is practically useless as the functional traits of the scarab swarm are its number of attacks, multi-charge potential and less so, its tarpitting ability. It loses all of these features at low base counts.

Scarabs also tend to overkill whatever they hit against a good opponent (because they will limit what the scarabs can actually get at, and since scarabs fold to fire after exploding a tank, adding more bases will just give you more overkill, not more effective power.

Also, addding more bases to a large swarm ups their target priority for the opponent. From what games I've seen, adding spyders tends to reduce the lifespan of your scarab unit because of this. So there's your use for a scarab-farm: drawing fire from other parts of the army. Unfortunately, the farm itself tends to take up a large part of the army, and the points would generally be better spent on a few small units and adding more effective power elsewhere.

- "It works for Nidz and it works for Necrons. "
Given that nids have almost no match-ups with even chances in the 5E codices (outside of SoB), using them as a yardstick for great mechanics isn't a sensible choice.

- "They can take down a lot of MEQ units too.", "A smart player is not going to bunch up all their scarabs to get pie plated by Manticores and Vindicators."
A smart player is not going to bunch up all their MeQ's to get multi-assaulted by scarabs. Assaults will cause your scarabs to become bunched up in any case. Especially those involving killing vehicles - their primary purpose. Also, no-one seriously thinks vindicators are good, and Manticores will take out quite a few scarabs (probably most of the unit) whether you're spread out or not. Remember, spreading out reduces your maneuverability and ability to move fast whilst maintaining cover drastically.

Your problem though, is that the model is not on the table when it is inside of the vehicle. This gives you nowhere to draw line of sight from. (No, the guy 'inside' a CCB is not your model, it is merely a part of the CCB model.)
Open topped transport vehicles are given a rule: "[...] passengers in an open-topped vehicle may fire, measuring range and line of sight from the hull of the vehicle [...]". Whilst it's impossible to tell whether open topped vehicles have fire points or not* it is clear that the sentence allowing models to fire from an open topped transport does just that. It lets them fire. Nowhere does it say that they can do anything else. It does not say "Models inside an open-topped vehicle may measure range and line of sight from the vehicle's hull."

In all honesty though, it doesn't matter what I say. The set of rules is so leaky and ill-defined that most in depth debates about the specifics will be ultimately unresolvable without direct intervention from GW. If interactions within the game were all built upwards from axiomatic mechanics, these issues would be solveable. Unfortunately, it isn't and they aren't.

* The sentence "There is no specific way to walk to the park." does not tell me whether or not I will actually be able to walk to the park.