[AoS Power Creep]

I'd say big imbalance and poorly designed units/armies are a bigger problem for casual and lower skilled players than competitive ones. Competitive players will usually just gravitate and focus on what is good and form less of an attachment to how an army is "meant" to play or what its lore is or anything else like that. If someone gets into AOS or any GW game for narrative and fluff-based reasons, starts collecting based on those reasons and then goes and finds out that that army or that style of play is completely unplayable trash, it's going to basically kill any enthusiasm they have to keep on playing.

[Are armies diverse enough in the TT?]

Buff stacking is an interesting one in that 40k is just as rife with it as AoS but yet again; there's still more diversity in armies.

Look at Marines and Sisters; strong armies that rely on and have access to large numbers of buffs and yet Sisters have some of the most diverse armies (in terms of total units being used) in the game currently and Marines see a lot of different unit types used as well, even though every Marine army ends up looking fairly similar despite Chapter differences. Like others said above, rule of 3 and stratagem limitations certainly play a part in this, but even then because 40k hasn't flattened its defensive profiles completely there still exist niches for different unit types to do damage. Intercessors can be buffed up to an insane amount, with bonuses to wound, re-rolls to hit, exploding hits on 6's, extra AP and multi-damage weapons. But at the end of the day, Plasma Inceptors kill elite armoured units better, as do Eradicators when it comes to vehicles/monsters and Aggressors are still superior anti-horde as well. Do you even really need "anti-" unit types outside of protection against spellcasters in AOS?

When it comes to "balancing" buff stacking and the like, Marines are still problematic but GW have done an excellent job with the new Necron codex. Core units (i.e. ones that benefit from the most buffs) are a relatively small number of units in total, however the non-core stuff is typically characterized by exceptional stat lines, solid special rules and most importantly; very good cost-efficiency. They benefit from fewer buffs, but they also require less to function and still have impact. Also, like Gloomspite Gitz, Necrons are essentially made up of 3 different armies: Core (Legions? idk what to call it), Canoptek and Destroyer Cult. Unlike GSG though you are not discouraged from taking the two other sub-types if your main focus is on one. A silver tide army, revolving around Core units and the Silent King can and will absolutely make use of Canoptek units and there also exist armies that are entirely composed of both Destroyer Cult and Canoptek.

Even looking at the "spammiest" top line army lists at the moment you have Orks. The competitive Ork list right now is Ghaz and an obscene and unreasonable amount of boyz, with 2 Mekboys with KFF's. Except those lists also include meganobz, mek guns, kommandos and trukks at least. So even in a "spam" list you're looking at 7+ unit types represented. That's obviously a small representation of the Codex and it's absolutely not a good thing, but it's still more unit types than you see in many AoS lists being run.

Both of those two above paragraphs are a combination of requiring different unit types to deal with different enemy ones, the necessity of certain units to achieve secondary objectives and just in general, subfaction and unit ability design is more in-depth than AOS in my opinion. Too often on AOS War Scrolls, units are basically just given bonuses or buffs that are around taking or receiving damage (specifically mortal wounds, which are a huge percentage of damage in AOS). This is one of the big issues with Idoneth after all; the entire army is just damage dealers, so you take the most efficient damage dealer in Eels. While utility abilities still exist, they're either too rare or too weak to really make an effect in most armies. Dankhold Troggoth's in GSG give you +1 Bravery to your Gobbos.... which is a massive "So?" If their damage or tankability was somehow needed or something a primarily goblin army couldn't achieve on its own then hey, they might have a niche. But as stated previously, you could conceivably just get by with goblins who could be better damage dealers and more resilient.

Battalions and subfactions also come into this as well, restricting army lists even further into specific niches and discouraging other unit types. Ymetrica Lumineth? Why take anything except Alarith units? No, seriously though. Add on an Alarith Temple Battalion too and it gets compounded even further. Something like the Novokh Dynasty in 40K Necrons however still leaves you open to plenty of unit types and adds a different dimension to the army without restricting it overly much. Obviously Novokh Doomsday Arks or Doom Scythes aren't gonna be great (since the trait is primarily close combat orientated) but plenty of non-combat units in Novokh THRIVE as the extra charge range, extra AP in combat and dynastic-specific stratagem give these units extra utility and damage that they otherwise lack.

[Are armies diverse enough in the TT?]

Decided to finally sign up after lurking for ages, hello everyone.

There's definitely a misconception I see from some in this thread that spamming is somewhat inevitable at higher levels of play, but even if you look at other GW games this isn't really the case. 40k manages to have a large amount of variety in units at top competitive levels of play and while spammy lists do exist for certain armies, it is by no means the standard. Space Marines, Sororitas, Necrons and Harlequins all have very diverse unit choices in their top lists just as an example. For Harlequins especially this is important because they're the most "AOS" of those factions, being that they have a relatively small model range, yet even then you see a more varied range of what small units the army has than you do in a lot of average competitive AOS lists. If Necrons were in AOS, their entire competitive build would probably be limited to TSK, 3 Chronomancers and 80-100 Warriors. Forget anything else in the Codex; that would be the build.

Enoby makes an excellent point about AOS's core mechanics reducing variety and diversity because if a unit of Night Goblins have an easy way to wound Behemoths or Elite Infantry on 2+ or 3+ then it completely discourages those types of units being used, while the Goblin player is basically pushed into taking nothing but basic gobbos. (Obviously this is just a broad example and not something competitively specific, but it highlights the point well. Morsarr Guard would be a more competitively apt example)

Battalions exacerbate the issue even further and it's weird that the system has continued to be deepened and expanded when Formations in 40k were rightly seen as a blight on the system. Battalions do at least have a points cost associated with them (which is good!) that helps keep them somewhat balanced, but it still encourages listbuilding styles that were part of what made 7th 40k so unenjoyable for people.

Different mission design would probably help AOS in this respect. Old ITC and now 9th edition missions for 40k provide lots of incentives to take varied and diverse armies, but like I said above, the flattened core statline mechanics of AOS will always remain an issue if they're not addressed in any way and the Battalion design will really just pigeonhole armies more and more. I'm honestly shocked sometimes looking at even "regular" AOS armies and how lacking in variety and diversity they are and to be completely clear this is not a result of small model ranges.