My best point is at the very end but I enjoyed talking about each item, sorry.
Sorry, I misunderstood--
I'd thought that that part of the assertion had been self-evident. Yes, obviously on the one hand you have "game mechanics", but purely fluffwise we know the following:
1. They began with small, light, infrequent attacks using only their weakest troops. They slowly scaled up the threat level and intensity of their attacks over time.
The attacks scale up rapidly, particularly in response to XCOM successfully stopping them. Which could be them adjusting the difficulty of the "test", but could also be them realizing the threat and committing more valuable resources and troops. That was the point of this:
* Abduct primitive hu-mans with expendable vat-grown scouts who still overcome all police and government forces.
* Oh what, this special forces team is confounding a portion of our efforts. Good thing all our weapons explode.
* Huh, lost some scout ships.
* ...They're doing *what* with the alloys? And they're figuring out elerium??
* Gloves come off, send in the cryssalids. The funding governments will collapse in anarchy. Loss of guinea pigs... unfortunate, but acceptable.
* Uh oh, that didn't work. Start mobilizing the honor guard (mutons) for ground assaults, rejects (floaters) first
* Uh oh, they found our base and *somehow reverse-engineered the key*. Commander down.
* Uh oh, the honor guard isn't stopping them, particularly since they've somehow reverse engineered our weapons. IN A CAVE! FROM SPARE PARTS!
* We'll just go down ourselves - Oh FUCK they're AWAKE
* And they built a bizarre McGuffin which beat our cloaking technology by hacking our psyberspace or some shit
* ... Maybe they'll negotiate?
* X_X
It doesn't have to be excused as only a gameplay thing, or explained as the aliens going easy on us.
2. Despite having psionic mind control which is demonstrably capable of functionally puppeteering human governments, they establish such holds on macroscopic power infrequently, slowly, and only in areas where XCOM repeatedly fails.
Their mind control is temporary and generally limited to line of sight. There's no evidence that they can puppeteer a human government... Nations leave the council, essentially surrendering, because they decide that XCOM cannot protect them.
It was very different in OldCOM, where psionics was more powerful. A battleship landed in the nation and forced them to withdraw. The NewCOM aliens just don't have that psionic power.
3. There aren't massive swaths of land completely overrun by Chryssalids, despite a certain mission demonstrating that it would be laughably easy to seed a few dozen locations with nascent hives.
Doing so would ruin their 7 billion test subjects, replacing them with a failed race.
4. Despite having a fleet apparently measured in scores if not hundreds of ships, the aliens launch them piecemeal, deploying only a few at a time, and scaling the threat similarly to their ground units.
Each ship type has a distinct mission. They don't send terror ships until they recognize XCOM's threat, and decide to disrupt their funding by forcing nations to leave. Supply ships are self-explanatory. Battleships are presumably precious, not to be wasted on mere abductions.
After all, XCOM can take down a battleship with jetfighters. In the first month or two, the aliens don't know to send the heavy stuff. After that, XCOM can take out their most powerful units... Even each battleship costs XCOM dearly, it may not seem worthwhile to the aliens.
5. As above, Meld Canisters. More specifically, their distribution. I think it's fair to argue that they're distributed intentionally to get Meld into human hands as part of the uplift process. Remember how Meld works? You get more when you've been doing poorly on missions and less when you've been doing well.
Into human hands, but not XCOM's. Whenever XCOM sees a meld canister, it's working to somehow disable itself. Why?
As I mentioned before, I think they're distributing meld as gaseous spores of nanites.
6. With the size of their fleet and the quality of their shipboard weapons, the aliens are easily capable of Independence Daying a few dozen major cities, which would undoubtedly make it easier to subdue humanity. They didn't.
We don't know the size of their fleet, or if it's being reinforced through a stargate on the Temple Ship.
Even if they do have the whole fleet, and have the weaponry to glass major cities, there are many arguments for why they may not:
They don't want to waste millions of test subject, preferring terror (thus, the terror ships and cryssalids)
If they send a battleship to destroy a city, the next battleship to approach a city will be nuked by humanity. Why not, when they're dead anyway?
Destroying whole cities outright could scare a country out of surrendering.
Basically the aliens are invaders, not destroyers. They need to occupy Earth, and that means scaring people yet offering them hope in surrender.
7. Connected to 4 & 6: Early on, you've got four interceptors and shit weapons. If they just wanted to conquer Earth, why didn't they launch a few dozen ships every time you sent out an interceptor? It would be painfully simple to utterly shut down XCOM's early air game even with half a dozen fighters, never mind a battleship or ten.
For one thing, shutting down the "early air game" only makes any sense because XCOM's funding and equipment are absurdly limited for gameplay reasons.
Lore wise, we can assume that similar jets are engaging scouts and harvesters worldwide. Maybe some are even succeeding. Heck, EXALT is getting concentrated Meld from *somewhere*, and XCOM sells/barters a lot of alien tech to the world governments.
With that much resistance, and if we can accept that they don't necessarily have infinite battleships, it makes sense to use the cheap nimble scouts and hold the big guns in reserve.
Do I need to go on?
Why "train" XCOM up, when they make their own supremely powerful psi-humans in their own labs?
Why did the aliens crush XCOM in the XCOM2 backstory, if they wanted to train XCOM? I say they had more resources, and XCOM wasn't good enough.
Actually, that is what's canon:
“We've all had our losses with XCOM, and so we use kind of a light touch. But the idea is that XCOM never made it out of conventional weapons; that the aliens came in overwhelming force and overwhelming numbers,” he said. “To say Impossible Iron Man is canon is perfect. We think that’s basically the experience that XCOM had.”
It's just Impossible Iron Man, so they still started with scouts and harvesters! XCOM just wasn't able to adapt and stop them. And they didn't ease off to train us up, they crushed the resistance and started their experiments.
*That* is what the aliens canonically did with their overwhelming resources.