I'm a 28 year old with an IQ of 130 and formations confuse the everliving shit out of me. The sheer pile of obtuse and random collections of special rules that are triggered by having non-related units taken in concert across multiple FOC slots and even factions or armies defies logic or rationale.
Lets take an incredibly simple one even, not even a complex rule - the new Dire Avengers formation from Craftworld Eldar.
Take three units of Dire Avengers. That's all you need to do.
You instantly gain three different special rules - one of which is one use only, one of which is a permanent change to your statline, and the other is a reroll to three different types of leadership test (but not the others).
My first problem with this is narrative. Why the hell are my Avengers suddenly so much more awesome just because there's 15 of them divided into three units than they are when there are 20 of them divided into two? For that matter why are they this much better than when taken in any other way? What mystical power is granted them that increases their accuracy, volume of fire, and morale simply by having three of them on the table, and not even needing to be fighting together? They could be at opposite ends of a 12ft table and it'd still work.
My second problem is that I need to now remember all of these abilities at different stages of the turn, and this gets exponentially harder the more units I have, from different formations, dataslates, codices et al. Even by sticking strictly to Codex: Eldar in a 2000pt army I can have something like 3 different sets of special rules above and beyond the actual units. Some of these occur at the start of turn, others during different phases, others are stat changes.
Thirdly, if I can barely keep track of this crap, how can I expect my opponent to? I fly some Fire Dragons at his tank, he works out in his head how many he can afford to leave alive based on the probability of 66% hit, 50% pen etc etc. It comes back around to my turn and I inform him that actually they're all BS5 and hitting 84% of the time, and rerolling armour pen rolls and adding one to the total... suddenly his maths is way off and I smoke something he didn't think I had a chance to. That's just one random example. Another would be overwatching at full BS or suddenly having a 12" run move etc etc.
Fourth, you can build some TRULY FUCKING STUPID armies with this. I'm not talking about the good old days of 10 Marines and 20 Terminators. I'm talking about armies made entirely out of Flyers, Superheavies, and other retarded crap that it's physically impossible for the bulk of an enemies army to meaningfully engage on any level. Why bother with Unbound when I can take an army made entirely out of Knights that can Skyfire, and therefore have no weakness of any kind whatsoever, while being functionally immune to 90% of weapons in the game, and impossible to delay, swamp, outmaneuver or outrun? What the hell happened to having a core of infantry to take and hold objectives, some vehicle support, and some firebase or outflanking elements? You know, strategy? An army that actually looks like, and functions like an army?
...nothing that cannot be countered one way or another.
Anything can be countered, but only by list tailoring. It is no longer possible to build an army that has a reasonable chance of success against all possible list combinations.
I never had a problem with the FOC. I had a problem with FOC *choking* where too many obviously good units were all in the same slot, or FOC starvation (the reverse) such as the old Eldar Codex with Heavy Support and FA respectively, but I never had a problem being required to take a core of infantry and a HQ while being free to build around that however I wanted without unbalancing myself too much. This current system, which makes HMRCs paperwork look positively simple, is one of maybe two/three reasons why I'm simply no longer playing 40k in any serious way, and have since moved onto X-Wing. I've even picked up Fantasy rather than play 40k regularly.